update(demo, content): refactor styles

* set demo-content `height:600px;` for properly layouts and scrolling.

src/components/content/demoBasicUsage/index.html

@@ -1,12 +1,12 @@
 
-<div ng-controller="AppCtrl" layout="column" style="padding-bottom: 15px;">
+<div ng-controller="AppCtrl" layout="column">
   <md-toolbar class="md-warn">
     <div class="md-toolbar-tools">
       <h2 class="md-flex">Toolbar: md-warn</h2>
     </div>
   </md-toolbar>
 
-  <md-content layout-padding style="height: 600px;padding: 24px;">
+  <md-content flex layout-padding>
     <p>Lorem ipsum dolor sit amet, ne quod novum mei. Sea omnium invenire mediocrem at, in lobortis conclusionemque nam. Ne deleniti appetere reprimique pro, inani labitur disputationi te sed. At vix sale omnesque, id pro labitur reformidans accommodare, cum labores honestatis eu. Nec quem lucilius in, eam praesent reformidans no. Sed laudem aliquam ne.</p>
 
     <p>