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 a45c1c1ee1e1941313d8b85a4c1bfaad51a9fd0f..fa39c223cd1c801ec46ddaa60baacedf4225c330 100644
--- a/cc/layer_tree_host.h
+++ b/cc/layer_tree_host.h
@@ -50,6 +50,7 @@ class PrioritizedResourceManager;
 class PrioritizedResource;
 class ResourceProvider;
 class ResourceUpdateQueue;
+class ScrollbarLayer;
 class HeadsUpDisplayLayer;
 class Region;
 struct ScrollAndScaleSet;
@@ -94,6 +95,7 @@ public:
     void layout();
     void beginCommitOnImplThread(LayerTreeHostImpl*);
     void finishCommitOnImplThread(LayerTreeHostImpl*);
+    void setPinchZoomScrollbarPropertiesIfNeeded();
     void willCommit();
     void commitComplete();
     scoped_ptr<OutputSurface> createOutputSurface();
@@ -152,6 +154,8 @@ 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);
@@ -244,6 +248,8 @@ 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;