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

third_party/WebKit/LayoutTests/compositing/layer-creation/scroll-partial-update-expected.txt

 scroll me
 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "Squashing Containment Layer",
       "shouldFlattenTransform": false
     },
     {
       "name": "LayoutBlockFlow DIV id='composited'",
       "position": [8, 8],
       "bounds": [20, 20]
     },
     {
       "name": "Squashing Layer (first squashed layer: LayoutBlockFlow (positioned) DIV id='overlay')",
       "position": [10, 8],
       "bounds": [400, 204],
       "drawsContent": true
-    },
-    {
-      "name": "LayoutBlockFlow DIV id='scroller'",
-      "position": [202, 10],
-      "bounds": [200, 200],
-      "shouldFlattenTransform": false,
-      "drawsContent": true,
-      "backgroundColor": "#FFFF00"
-    },
-    {
-      "name": "Scrolling Layer",
-      "bounds": [185, 200],
-      "shouldFlattenTransform": false
-    },
-    {
-      "name": "Scrolling Contents Layer",
-      "bounds": [185, 400],
-      "contentsOpaque": true,
-      "drawsContent": true
-    },
-    {
-      "name": "Overflow Controls Host Layer",
-      "bounds": [200, 200]
-    },
-    {
-      "name": "Vertical Scrollbar Layer",
-      "position": [185, 0],
-      "bounds": [15, 200]
     }
   ]
 }