Introduce bottom controls to CC and let it respond to scrolling

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 7581ff2cac9f128d6f45f4188f076f741ceea9a5..77d55619a2a54bd4b619231da238944a45adbe04 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -251,6 +251,7 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   void SetViewportSize(const gfx::Size& device_viewport_size);
   void SetTopControlsHeight(float height, bool shrink);
   void SetTopControlsShownRatio(float ratio);
+  void setBottomControlHeight(float height);
 
   gfx::Size device_viewport_size() const { return device_viewport_size_; }
 
@@ -546,6 +547,7 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   bool top_controls_shrink_blink_size_;
   float top_controls_height_;
   float top_controls_shown_ratio_;
+  float bottom_controls_height_;
   float device_scale_factor_;
   float painted_device_scale_factor_;