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

cc/layer_tree_host_impl.h

Index: cc/layer_tree_host_impl.h
diff --git a/cc/layer_tree_host_impl.h b/cc/layer_tree_host_impl.h
index f59108087c69209783a0f69da546abbd31e5df69..e4941aa9d41930fb292c1e70e31efeda20b4871f 100644
--- a/cc/layer_tree_host_impl.h
+++ b/cc/layer_tree_host_impl.h
@@ -11,6 +11,7 @@
 #include "cc/animation_events.h"
 #include "cc/cc_export.h"
 #include "cc/input_handler.h"
+#include "cc/layer_sorter.h"
 #include "cc/output_surface_client.h"
 #include "cc/render_pass.h"
 #include "cc/render_pass_sink.h"
@@ -27,6 +28,7 @@ class FrameRateCounter;
 class LayerImpl;
 class LayerTreeHostImplTimeSourceAdapter;
 class LayerTreeImpl;
+class PinchZoomScrollbarsManager;
 class PageScaleAnimation;
 class RenderPassDrawQuad;
 class ResourceProvider;
@@ -255,7 +257,7 @@ public:
 
     void renderingStats(RenderingStats*) const;
 
-    void updateRootScrollLayerImplTransform();
+    void updateImplTransformOnRootChildren();
 
     void sendManagedMemoryStats(
        size_t memoryVisibleBytes,
@@ -355,6 +357,10 @@ private:
 
     scoped_ptr<LayerTreeImpl> m_activeTree;
 
+    scoped_ptr<LayerImpl> m_rootLayerImpl;
+    LayerImpl* m_rootScrollLayerImpl;
+    LayerImpl* m_currentlyScrollingLayerImpl;
+    int m_scrollingLayerIdFromPreviousTree;
     bool m_scrollDeltaIsInViewportSpace;
     LayerTreeSettings m_settings;
     LayerTreeDebugState m_debugState;