Clamp compositor's top controls shown ratio on commit from main thread.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 56d9c6c4f2862ce984e704328694916d57bc4e1f..de16100cd5a727db95c3820b25a4770691bc377b 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -722,12 +722,19 @@ void LayerTreeImpl::set_top_controls_height(float top_controls_height) {
     layer_tree_host_impl_->UpdateViewportContainerSizes();
 }
 
-bool LayerTreeImpl::SetCurrentTopControlsShownRatio(float ratio) {
+bool LayerTreeImpl::ClampTopControlsShownRatio() {
+  float ratio = top_controls_shown_ratio_->Current(true);
   ratio = std::max(ratio, 0.f);
   ratio = std::min(ratio, 1.f);
   return top_controls_shown_ratio_->SetCurrent(ratio);
 }
 
+bool LayerTreeImpl::SetCurrentTopControlsShownRatio(float ratio) {
+  bool changed = top_controls_shown_ratio_->SetCurrent(ratio);
+  changed |= ClampTopControlsShownRatio();
+  return changed;
+}
+
 void LayerTreeImpl::PushTopControlsFromMainThread(
     float top_controls_shown_ratio) {
   PushTopControls(&top_controls_shown_ratio);
@@ -741,7 +748,9 @@ void LayerTreeImpl::PushTopControls(const float* top_controls_shown_ratio) {
     top_controls_shown_ratio_->PushFromMainThread(*top_controls_shown_ratio);
   }
   if (IsActiveTree()) {
-    if (top_controls_shown_ratio_->PushPendingToActive())
+    bool changed_active = top_controls_shown_ratio_->PushPendingToActive();
+    changed_active |= ClampTopControlsShownRatio();
+    if (changed_active)
       layer_tree_host_impl_->DidChangeTopControlsPosition();
   }
 }