LazyLoad: Merge scroll.html into image-loading-lazy.tentative.html

The two tests do the same thing but scroll.html checks for placeholder, so move
that functionality into image-loading-lazy.tentative.html and remove scroll.html.

Bug: 1031157
Change-Id: Id0b15fb1a2866933ecaa072a10f4fbc8655d033c

loading/lazyload/image-loading-lazy.tentative.html

@@ -5,6 +5,7 @@
   <link rel="help" href="https://github.com/scott-little/lazyload">
   <script src="/resources/testharness.js"></script>
   <script src="/resources/testharnessreport.js"></script>
+  <script src="common.js"></script>
 </head>
 
 <!--
@@ -24,12 +25,16 @@
 
   window.addEventListener("load", t.step_func(function() {
     assert_true(has_in_viewport_loaded, "The in_viewport element should have loaded before window.load().");
+    assert_true(document.getElementById("in_viewport").complete);
     assert_false(has_window_loaded, "The window.load() event should only fire once.");
     has_window_loaded = true;
     document.getElementById("below_viewport").scrollIntoView();
   }));
 
   const below_viewport_img_onload = t.step_func_done(function() {
+    assert_true(is_image_fully_loaded(
+                  document.getElementById("below_viewport"),
+                  document.getElementById("in_viewport")));
     assert_true(has_window_loaded, "The window.load() event should have fired before below_viewport loaded.");
   });
 </script>