Introduce bottom controls to CC and let it respond to scrolling

cc/trees/layer_tree.cc

Index: cc/trees/layer_tree.cc
diff --git a/cc/trees/layer_tree.cc b/cc/trees/layer_tree.cc
index bbce951aad05cb2fc36f68a1d837122922e7c027..60718316f285ca9c3d478c1599db64b3788bae56 100644
--- a/cc/trees/layer_tree.cc
+++ b/cc/trees/layer_tree.cc
@@ -45,6 +45,7 @@ LayerTree::Inputs::Inputs()
     : top_controls_height(0.f),
       top_controls_shown_ratio(0.f),
       top_controls_shrink_blink_size(false),
+      bottom_controls_height(0.f),
       device_scale_factor(1.f),
       painted_device_scale_factor(1.f),
       page_scale_factor(1.f),
@@ -173,6 +174,14 @@ void LayerTree::SetTopControlsShownRatio(float ratio) {
   SetNeedsCommit();
 }
 
+void LayerTree::SetBottomControlsHeight(float height) {
+  if (inputs_.bottom_controls_height == height)
+    return;
+
+  inputs_.bottom_controls_height = height;
+  SetNeedsCommit();
+}
+
 void LayerTree::SetPageScaleFactorAndLimits(float page_scale_factor,
                                             float min_page_scale_factor,
                                             float max_page_scale_factor) {
@@ -384,6 +393,7 @@ void LayerTree::PushPropertiesTo(LayerTreeImpl* tree_impl) {
   tree_impl->set_top_controls_shrink_blink_size(
       inputs_.top_controls_shrink_blink_size);
   tree_impl->set_top_controls_height(inputs_.top_controls_height);
+  tree_impl->set_bottom_controls_height(inputs_.bottom_controls_height);
   tree_impl->PushTopControlsFromMainThread(inputs_.top_controls_shown_ratio);
   tree_impl->elastic_overscroll()->PushFromMainThread(elastic_overscroll_);
   if (tree_impl->IsActiveTree())