Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/layer_tree_host.h

Index: cc/layer_tree_host.h
diff --git a/cc/layer_tree_host.h b/cc/layer_tree_host.h
index 3407dba4b20ffbe5d362ca44fece2e5ec2b6652c..b57e802f74da48acaed91c6cfe1cfa24873e705f 100644
--- a/cc/layer_tree_host.h
+++ b/cc/layer_tree_host.h
@@ -53,6 +53,7 @@ class PrioritizedResource;
 class Region;
 class ResourceProvider;
 class ResourceUpdateQueue;
+class ScrollbarLayer;
 class TopControlsManager;
 struct ScrollAndScaleSet;
 
@@ -97,6 +98,8 @@ public:
     void layout();
     void beginCommitOnImplThread(LayerTreeHostImpl*);
     void finishCommitOnImplThread(LayerTreeHostImpl*);
+    void setPinchZoomScrollbarPropertiesIfNeeded();
+    bool rootScrollLayerUsesOverlayScrollbars() const;
     void willCommit();
     void commitComplete();
     scoped_ptr<OutputSurface> createOutputSurface();
@@ -155,6 +158,8 @@ public:
     const Layer* rootLayer() const { return m_rootLayer.get(); }
     void setRootLayer(scoped_refptr<Layer>);
 
+    Layer* rootScrollLayer() const;
+
     const LayerTreeSettings& settings() const { return m_settings; }
 
     void setDebugState(const LayerTreeDebugState& debugState);
@@ -253,6 +258,8 @@ private:
 
     scoped_refptr<Layer> m_rootLayer;
     scoped_refptr<HeadsUpDisplayLayer> m_hudLayer;
+    scoped_refptr<ScrollbarLayer> m_pinchZoomScrollbarHorizontal;
+    scoped_refptr<ScrollbarLayer> m_pinchZoomScrollbarVertical;
 
     scoped_ptr<PrioritizedResourceManager> m_contentsTextureManager;
     scoped_ptr<PrioritizedResource> m_surfaceMemoryPlaceholder;