Support high DPI scrollbar on top level web frame.

cc/LayerChromium.h

Index: cc/LayerChromium.h
diff --git a/cc/LayerChromium.h b/cc/LayerChromium.h
index 0b4fe63437cc334ece42681dd94c9e4e4fafc6d4..6f8a147a03eb815de1775eceabc401719728c0bc 100644
--- a/cc/LayerChromium.h
+++ b/cc/LayerChromium.h
@@ -230,6 +230,7 @@ public:
     void setContentsScale(float);
 
     // When true, the layer's contents are not scaled by the current page scale factor.
+    // setBoundsContainPageScale recursively sets the value on all child layers.
     void setBoundsContainPageScale(bool);
     bool boundsContainPageScale() const { return m_boundsContainPageScale; }
 
@@ -271,6 +272,8 @@ protected:
 
     void setNeedsCommit();
 
+    IntRect layerRectToContentRect(const WebKit::WebRect& layerRect);
+
     // This flag is set when layer need repainting/updating.
     bool m_needsDisplay;