Disable overlay scrollbars in Blink when hidden by the compositor.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 5b90fd12c5a679f7c2823aa1327898fe84ed435e..0fa98658b739108c3a736a1e65eb55a298217c2f 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2220,6 +2220,11 @@ LayerImpl* LayerTreeHostImpl::ViewportMainScrollLayer() {
   return viewport()->MainScrollLayer();
 }
 
+void LayerTreeHostImpl::SetScrollbarsHidden(int layer_id, bool hidden) {
+  scrollbars_hidden_map_[layer_id] = hidden;
+  client_->SetNeedsCommitOnImplThread();

[aelias_OOO_until_Jul13, 2016/10/26 19:01:03]

Please check if any of the map entries changed and only SetNeedsCommit if so.

[bokan, 2016/10/27 20:56:46]

The map is cleared on each commit so that it contains only layers with changed
scrollbar visibility (to avoid iterating over all scroll layers each commit). We
could remove existing and non-equal entries rather than flipping the bit since
that means we've changed back and forth without a commit, but SetNeedsCommit
will have already been set by the first call.

Since this flag is synced one-way only, we don't have a record of what the main
thread thinks this value is so we can't compare against that either.

I've added this bit to LayerImpl so we can avoid spamming NeedsCommit if nothing
on impl has changed.

[aelias_OOO_until_Jul13, 2016/10/27 21:22:09]

OK.  Though with that bit added to LayerImpl, the scrollbars_hidden_map_ state
isn't needed anymore.  Please just put scrollbar visibility boolean into
info->scrollbars on every commit.  The early-return on the main thread side will
take care of avoiding substantive work from that.

+}
+
 void LayerTreeHostImpl::CleanUpTileManagerAndUIResources() {
   ClearUIResources();
   tile_manager_.FinishTasksAndCleanUp();
@@ -3346,14 +3351,26 @@ static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
           ? tree_impl->InnerViewportScrollLayer()->id()
           : Layer::INVALID_ID;
 
-  return tree_impl->property_trees()->scroll_tree.CollectScrollDeltas(
+  tree_impl->property_trees()->scroll_tree.CollectScrollDeltas(
       scroll_info, inner_viewport_layer_id);
 }
 
+static void CollectScrollbarUpdates(
+    ScrollAndScaleSet* scroll_info,
+    std::unordered_map<int, bool>* scrollbars_hidden_map) {
+  scroll_info->scrollbars.reserve(scrollbars_hidden_map->size());
+  for (auto& pair : *scrollbars_hidden_map) {
+    scroll_info->scrollbars.push_back(
+        LayerTreeHostCommon::ScrollbarsUpdateInfo(pair.first, pair.second));
+  }
+  scrollbars_hidden_map->clear();
+}
+
 std::unique_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   std::unique_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
   CollectScrollDeltas(scroll_info.get(), active_tree_.get());
+  CollectScrollbarUpdates(scroll_info.get(), &scrollbars_hidden_map_);
   scroll_info->page_scale_delta =
       active_tree_->page_scale_factor()->PullDeltaForMainThread();
   scroll_info->top_controls_delta =