fix(mdSubheader): Non top-level md-content causes incorrect position.

If the subheader's parent `<md-content>` was not the top-level item,
the stickied subheader would be incorrectly positioned further down
the page.

Also added more tests to toolbar and fixed issue to ensure subheader
works with a toolbar using `md-scroll-shrink`.

Fixes angular#4420. References angular#2825. Closes angular#4439.

src/components/sticky/sticky.js

@@ -70,7 +70,6 @@ function MdSticky($document, $mdConstant, $$rAF, $mdUtil) {
     contentEl.on('$scroll', onScroll);
 
     var self;
-    var stickyBaseoffset = contentEl.prop('offsetTop');
     return self = {
       prev: null,
       current: null, //the currently stickied item
@@ -86,7 +85,6 @@ function MdSticky($document, $mdConstant, $$rAF, $mdUtil) {
     // Add an element and its sticky clone to this content's sticky collection
     function add(element, stickyClone) {
       stickyClone.addClass('md-sticky-clone');
-      stickyClone.css('top', stickyBaseoffset + 'px');
 
       var item = {
         element: element,

src/components/subheader/demoBasicUsage/index.html

@@ -1,5 +1,9 @@
-
 <div ng-controller="SubheaderAppCtrl" layout="column" flex layout-fill>
+
+  <md-toolbar md-scroll-shrink>
+    <div class="md-toolbar-tools">My Messages</div>
+  </md-toolbar>
+
   <md-content style="height: 600px;" md-theme="altTheme">
 
     <section>

src/components/toolbar/toolbar.js

@@ -68,7 +68,9 @@ function mdToolbarDirective($$rAF, $mdConstant, $mdUtil, $mdTheming, $animate) {
 
       $mdTheming(element);
 
-      setupScrollShrink();
+      if (angular.isDefined(attr.mdScrollShrink)) {
+        setupScrollShrink();
+      }
 
       function setupScrollShrink() {
 
@@ -100,7 +102,7 @@ function mdToolbarDirective($$rAF, $mdConstant, $mdUtil, $mdTheming, $animate) {
         // If the scope is destroyed (which could happen with ng-if), make sure
         // to disable scroll shrinking again
 
-        scope.$on('$destroy', disableScrollShrink );
+        scope.$on('$destroy', disableScrollShrink);
 
         /**
          *
@@ -115,9 +117,15 @@ function mdToolbarDirective($$rAF, $mdConstant, $mdUtil, $mdTheming, $animate) {
             onMdContentLoad(null, closestContent);
           }
 
-          // Disable only if the attribute's expression evaluates to false
+          // Evaluate the expression
+          shrinkWithScroll = scope.$eval(shrinkWithScroll);
 
-         if ( shrinkWithScroll )  disableScrollShrink = enableScrollShrink();
+          // Disable only if the attribute's expression evaluates to false
+          if (shrinkWithScroll === false) {
+            disableScrollShrink();
+          } else {
+            disableScrollShrink = enableScrollShrink();
+          }
         }
 
         /**
@@ -170,7 +178,7 @@ function mdToolbarDirective($$rAF, $mdConstant, $mdUtil, $mdTheming, $animate) {
          *
          */
         function enableScrollShrink() {
-          if ( !contentElement )     return angular.noop;           // no md-content
+          if (!contentElement)     return angular.noop;           // no md-content
 
           contentElement.on('scroll', debouncedContentScroll);
           contentElement.attr('scroll-shrink', 'true');

src/components/toolbar/toolbar.spec.js

@@ -131,6 +131,17 @@ describe('<md-toolbar>', function() {
     expect($exceptionHandler.errors).toEqual([]);
   }));
 
+  it('disables scroll shrink when the attribute is not provided', inject(function() {
+    build(
+      '<div>' +
+      '  <md-toolbar></md-toolbar>' +
+      '  <md-content></md-content>' +
+      '</div>'
+    );
+
+    expect(element.find('md-content').attr('scroll-shrink')).toEqual(undefined);
+  }));
+
   it('enables scroll shrink when the attribute has no value', function() {
     build(
       '<div>' +
@@ -142,9 +153,37 @@ describe('<md-toolbar>', function() {
     expect(element.find('md-content').attr('scroll-shrink')).toEqual('true');
   });
 
-  function build(template) {
+  it('disables scroll shrink if the expression evaluates to false', function() {
+    var pageScope = $rootScope.$new();
+
+    // Set the value to false
+    pageScope.$apply('someValue = false');
+
+    // Build the element
+    build(
+      // Pass our template
+      '<div>' +
+      '  <md-toolbar md-scroll-shrink="someValue"></md-toolbar>' +
+      '  <md-content></md-content>' +
+      '</div>',
+
+      // Pass our custom pageScope
+      pageScope
+    );
+
+    // Check that scroll shrink is disabled
+    expect(element.find('md-content').attr('scroll-shrink')).toEqual('false');
+  });
+
+
+  function build(template, scope) {
     inject(function($compile) {
-      pageScope = $rootScope.$new();
+      if (scope) {
+        pageScope = scope
+      } else {
+        pageScope = $rootScope.$new();
+      }
+
       element = $compile(template)(pageScope);
       controller = element.controller('mdToolbar');