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..83e94efa50869703ca70d77e771b109fbe5d35da 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 +191,26 @@ 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();
+
+  enum PinchZoomScrollbarUpdateReason {

[enne (OOO), 2013/03/06 19:32:58]

What's the purpose of this enum? Are you just emulating the updateDrawProperties
update reason? Is updating 6 properties so expensive that you have to guard
against doing it too often?

[wjmaclean, 2013/03/06 22:36:53]

I was hoping that it might reduce the number of paint()'s issue to the PZ
scrollbars, though I can't really say that it helps much. It's gone.

[enne (OOO), 2013/03/06 22:45:38]

Paints? Isn't all the painting done on the main thread?

[wjmaclean, 2013/03/07 22:57:39]

Yes, but I was concerned about updates on the impl-side parameters possibly
making their way back and causing re-paints on the main thread ... perhaps I was
being overly cautious.

[enne (OOO), 2013/03/07 23:41:17]

I think you're being overly cautious.  I don't see how UpdatePinchZoomScrollbars
could cause a repaint.

+    UPDATE_IMPL_THREAD_SCROLL,
+    UPDATE_MAIN_THREAD_SCROLL
+  };
+
+  void UpdatePinchZoomScrollbarsIfNeeded(
+    PinchZoomScrollbarUpdateReason update_reason);
+  void FadeInPinchZoomScrollbars();
+  void FadeOutPinchZoomScrollbars();
+
+  bool HasPinchZoomScrollbars() {
+    return pinch_zoom_scrollbar_horizontal_layer_id &&
+           pinch_zoom_scrollbar_vertical_layer_id;
+  }
+
 protected:
   LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl);
 
@@ -202,6 +223,9 @@ protected:
   SkColor background_color_;
   bool has_transparent_background_;
 
+  int pinch_zoom_scrollbar_horizontal_layer_id;

[enne (OOO), 2013/03/06 19:32:58]

style nit: member variables end with trailing underscores.

[wjmaclean, 2013/03/06 22:36:53]

Thanks, missed that!

Done.

+  int pinch_zoom_scrollbar_vertical_layer_id;
+
   float page_scale_factor_;
   float page_scale_delta_;
   float sent_page_scale_delta_;