Introduce bottom controls to CC and let it respond to scrolling

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 6fdc804b3cf1e300642b1e75b6cdfce1b79d8990..c72e6f781a722e62b3349de2a141c6860e3272ec 100644
--- a/content/renderer/gpu/render_widget_compositor.h
+++ b/content/renderer/gpu/render_widget_compositor.h
@@ -154,6 +154,9 @@ class CONTENT_EXPORT RenderWidgetCompositor
                               bool animate) override;
   void setTopControlsHeight(float height, bool shrink) override;
   void setTopControlsShownRatio(float) override;
+  // TODO(ianwen): Move this method to WebLayerTreeView and implement main
+  // thread scrolling.
+  virtual void setBottomControlHeight(float height);
 
   // cc::LayerTreeHostClient implementation.
   void WillBeginMainFrame() override;