Make compositor and blink talk in fractional scroll offset

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 1039 matching lines @@
         m_client->notifyAnimationStarted(this, monotonicTime);
 }
 
 void GraphicsLayer::notifyAnimationFinished(double, WebCompositorAnimation::TargetProperty)
 {
     // Do nothing.
 }
 
 void GraphicsLayer::didScroll()
 {
-    if (m_scrollableArea)
-        m_scrollableArea->scrollToOffsetWithoutAnimation(m_scrollableArea->minimumScrollPosition() + toIntSize(m_layer->layer()->scrollPosition()));
+    if (m_scrollableArea) {
+        DoublePoint newPosition = m_scrollableArea->minimumScrollPosition() + toDoubleSize(m_layer->layer()->scrollPositionDouble());
+        m_scrollableArea->scrollToOffsetWithoutAnimation(toFloatPoint(newPosition));

[Rick Byers, 2014/10/04 00:04:24]

FIXME to remove the 'toFloatPoint' (like all the other callsites).  Hopefully
that's an easy cleanup to do soon (so we can stop adding new callsites).

[Yufeng Shen (Slow to review), 2014/10/06 17:54:58]

Done.

+    }
 }
 
 } // namespace blink
 
 #ifndef NDEBUG
 void showGraphicsLayerTree(const blink::GraphicsLayer* layer)
 {
     if (!layer)
         return;
 
     String output = layer->layerTreeAsText(blink::LayerTreeIncludesDebugInfo);
     fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif