Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

content/renderer/gpu/render_widget_compositor.h

Index: content/renderer/gpu/render_widget_compositor.h
diff --git a/content/renderer/gpu/render_widget_compositor.h b/content/renderer/gpu/render_widget_compositor.h
index 9f11b25a8989197f851198dd85bf2a9b78596620..c29ac08268f315109ac04aff192344f694924953 100644
--- a/content/renderer/gpu/render_widget_compositor.h
+++ b/content/renderer/gpu/render_widget_compositor.h
@@ -84,6 +84,14 @@ class RenderWidgetCompositor : public WebKit::WebLayerTreeView,
   virtual void finishAllRendering();
   virtual void setDeferCommits(bool defer_commits);
   virtual void registerForAnimations(WebKit::WebLayer* layer);
+  virtual void registerPinchViewportLayers(
+      const WebKit::WebLayer* innerViewportClipLayer,
+      const WebKit::WebLayer* pageScaleLayerLayer,

[trchen, 2013/06/20 01:16:08]

nits: LayerLayer?

[wjmaclean, 2013/06/21 18:20:53]

Done.

+      const WebKit::WebLayer* innerViewportScrollLayer,
+      const WebKit::WebLayer* outerViewportScrollLayer,
+      const WebKit::WebLayer* innerViewportHorizontalScrollbarLayer,
+      const WebKit::WebLayer* innerViewportVerticalScrollbarLayer) OVERRIDE;

[trchen, 2013/06/20 01:16:08]

I feel CC is making too much assumption about the layer structure.

It looks like the main purpose of this is to allow CC to setup the scrollbar
layers. From the other thread I know that you and jamesr@ made the consensus.
Just wondering why? Is it really messy to setup the scrollbar layers from
Blink-side? Does that imply we want to setup scrollbar layers in CC for frames
and overflow:scroll as well?

[wjmaclean, 2013/06/21 18:20:53]

This is a special case for scrollbars that (1) Blink never needs to interact
with, and (2) have no associated WebCore::Scrollbar associated with them. In
particular, if we instantiate on the Blink side (as I originally proposed), then
(2) means we need additional plumbing though the compositor bindings, since we
need another pathway to create scrollbars when we don't have a backing
WebCore::Scrollbar.

[trchen, 2013/06/24 21:38:04]

Thanks for the explaination!

+  virtual void clearPinchViewportLayers() OVERRIDE;
   virtual void renderingStats(WebKit::WebRenderingStats& stats) const {}
   virtual void setShowFPSCounter(bool show);
   virtual void setShowPaintRects(bool show);