Introduce bottom controls to CC and let it respond to scrolling

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 05350ce2a566970523355c5e45a6225c8ac90586..1ea7a9e8988b4249ac24b8c2c5bcbb783cec0cba 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -418,6 +418,8 @@ class CC_EXPORT LayerTreeImpl {
   void set_top_controls_height(float top_controls_height);
   float top_controls_height() const { return top_controls_height_; }
   void PushTopControlsFromMainThread(float top_controls_shown_ratio);
+  void set_bottom_controls_height(float bottom_controls_height);
+  float bottom_controls_height() const { return bottom_controls_height_; }
 
   void SetPendingPageScaleAnimation(
       std::unique_ptr<PendingPageScaleAnimation> pending_animation);
@@ -579,6 +581,7 @@ class CC_EXPORT LayerTreeImpl {
   // controls at the time of the last layout.
   bool top_controls_shrink_blink_size_;
   float top_controls_height_;
+  float bottom_controls_height_;
 
   // The amount that the top controls are shown from 0 (hidden) to 1 (fully
   // shown).