Introduce bottom controls to CC and let it respond to scrolling

cc/trees/layer_tree.h

Index: cc/trees/layer_tree.h
diff --git a/cc/trees/layer_tree.h b/cc/trees/layer_tree.h
index 6f1f916372511a39b872829814bd86500b917738..5b1fe50b7cd9a1abf6c7e6de7fdb1969414adeff 100644
--- a/cc/trees/layer_tree.h
+++ b/cc/trees/layer_tree.h
@@ -87,6 +87,7 @@ class CC_EXPORT LayerTree {
 
   void SetTopControlsHeight(float height, bool shrink);
   void SetTopControlsShownRatio(float ratio);
+  void SetBottomControlsHeight(float height);
 
   void SetPageScaleFactorAndLimits(float page_scale_factor,
                                    float min_page_scale_factor,
@@ -176,6 +177,8 @@ class CC_EXPORT LayerTree {
     float top_controls_shown_ratio;
     bool top_controls_shrink_blink_size;
 
+    float bottom_controls_height;
+
     float device_scale_factor;
     float painted_device_scale_factor;
     float page_scale_factor;