Reland "Only promote opaque scrollers which are stacking contexts."

third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-background-transparent-to-opaque.html

 <script src="../../resources/run-after-layout-and-paint.js"></script>
 <script src="resources/composited-scroll.js"></script>
 <script>
     if (window.internals)
         internals.runtimeFlags.compositeOpaqueScrollersEnabled = true;
 
     if (window.testRunner) {
         testRunner.dumpAsTextWithPixelResults();
         testRunner.waitUntilDone();
     }
@@ skipping 20 matching lines @@
                 result += "Fail.\n";
             document.getElementById("test-2-output").innerHTML = result;
             runAfterLayoutAndPaint(function() { testRunner.notifyDone(); });
         });
     }
 </script>
 <style>
 #scroller {
     background: transparent local content-box;
     border: 10px solid rgba(0, 255, 0, 0.5);
+    contain: paint;
     overflow: scroll;
     width: 200px;
     height: 200px;
 }
 .spacer {
     height: 300px;
 }
 </style>
 <div id="scroller"><div class="spacer"></div></div>
 <p>Scroller background should be green</p>
 <p id="test-1-output"></p>
 <p id="test-2-output"></p>