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 cd10771b1b2c8e09f521c6b16e2289b919e9750a..5e875f9e69072b975dbcb02b0d59685d708ae77e 100644
--- a/content/renderer/gpu/render_widget_compositor.h
+++ b/content/renderer/gpu/render_widget_compositor.h
@@ -161,6 +161,9 @@ 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 setBottomControlsHeight(float height);
 
   // cc::LayerTreeHostClient implementation.
   void WillBeginMainFrame() override;