Made layout viewport scroll updates from compositor work like ordinary layers.

third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1205 matching lines @@
     linkHighlight->layer()->setLayerClient(this);
     updateChildList();
 }
 
 void GraphicsLayer::removeLinkHighlight(LinkHighlight* linkHighlight)
 {
     m_linkHighlights.remove(m_linkHighlights.find(linkHighlight));
     updateChildList();
 }
 
-void GraphicsLayer::setScrollableArea(ScrollableArea* scrollableArea, bool isViewport)
+void GraphicsLayer::setScrollableArea(ScrollableArea* scrollableArea, bool isVisualViewport)
 {
     if (m_scrollableArea == scrollableArea)
         return;
 
     m_scrollableArea = scrollableArea;
 
-    // Viewport scrolling may involve pinch zoom and gets routed through
-    // WebViewImpl explicitly rather than via GraphicsLayer::didScroll.
-    if (isViewport)
+    // VisualViewport scrolling may involve pinch zoom and gets routed through
+    // WebViewImpl explicitly rather than via GraphicsLayer::didScroll since it
+    // needs to be set in tandem with the page scale delta.
+    if (isVisualViewport)
         m_layer->layer()->setScrollClient(0);
     else
         m_layer->layer()->setScrollClient(this);
 }
 
 void GraphicsLayer::didScroll()
 {
     if (m_scrollableArea) {
         DoublePoint newPosition = m_scrollableArea->minimumScrollPosition() + toDoubleSize(m_layer->layer()->scrollPositionDouble());
 
@@ skipping 133 matching lines @@
 {
     if (!layer) {
         fprintf(stderr, "Cannot showGraphicsLayerTree for (nil).\n");
         return;
     }
 
     String output = layer->layerTreeAsText(blink::LayerTreeIncludesDebugInfo);
     fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif