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

cc/layer_tree_impl.h

Index: cc/layer_tree_impl.h
diff --git a/cc/layer_tree_impl.h b/cc/layer_tree_impl.h
index ee2b6df0491647cee45fcc75c78e5b431b4949b1..66b161b62d951ae1c9b58c2ccecb39b8424c6965 100644
--- a/cc/layer_tree_impl.h
+++ b/cc/layer_tree_impl.h
@@ -34,6 +34,7 @@ class OutputSurface;
 class PaintTimeCounter;
 class Proxy;
 class ResourceProvider;
+class ScrollbarLayerImpl;
 class TileManager;
 struct RendererCapabilities;
 
@@ -190,6 +191,19 @@ class CC_EXPORT LayerTreeImpl {
   // Useful for debug assertions, probably shouldn't be used for anything else.
   Proxy* proxy() const;
 
+  void SetPinchZoomHorizontalLayerId(int layer_id);
+  ScrollbarLayerImpl* PinchZoomScrollbarHorizontal();
+  void SetPinchZoomVerticalLayerId(int layer_id);
+  ScrollbarLayerImpl* PinchZoomScrollbarVertical();
+
+  void UpdatePinchZoomScrollbarsIfNeeded();
+  void SetPinchZoomScrollbarsVisibility();
+
+  bool HasPinchZoomScrollbars() {
+    return pinch_zoom_scrollbar_horizontal_layer_id &&
+           pinch_zoom_scrollbar_vertical_layer_id;
+  }
+
 protected:
   LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl);
 
@@ -202,6 +216,9 @@ protected:
   SkColor background_color_;
   bool has_transparent_background_;
 
+  int pinch_zoom_scrollbar_horizontal_layer_id;
+  int pinch_zoom_scrollbar_vertical_layer_id;
+
   float page_scale_factor_;
   float page_scale_delta_;
   float sent_page_scale_delta_;