Disable overlay scrollbars in Blink when hidden by the compositor.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 1abdaebf90676feafe9cbd9c35e7bb2ff0ff947d..b54b136ab8f555457a028b54ad25a1b26f97b5d1 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -97,6 +97,7 @@ Layer::Layer()
       force_render_surface_for_testing_(false),
       subtree_property_changed_(false),
       may_contain_video_(false),
+      scrollbars_hidden_(false),

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

This state is redundant with the one in ScrollableArea, please delete it.  This
would mean getting rid of the early-return in
Layer::SetScrollbarsHiddenFromImplSide().  There's already early-return coverage
in ScrollableArea::didChangeScrollbarsHidden() and you just need to add
analogous early-return to FrameView::didChangeScrollbarsHidden().

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

Done.
FrameView::didChangeScrollbarsHidden and
PaintLayerScrollableArea::didChangeScrollbarsHidden are only called from
ScrollableArea::setScrollbarsHidden (that's what you meant above) if there's a
change so they don't need an early return.

       safe_opaque_background_color_(0),
       draw_blend_mode_(SkXfermode::kSrcOver_Mode),
       num_unclipped_descendants_(0) {}
@@ -825,6 +826,7 @@ void Layer::SetScrollOffsetFromImplSide(
 
   if (!inputs_.did_scroll_callback.is_null())
     inputs_.did_scroll_callback.Run();
+

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

nit: unnecessary newline

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

Done.

   // The callback could potentially change the layer structure:
   // "this" may have been destroyed during the process.
 }
@@ -1391,6 +1393,7 @@ void Layer::LayerSpecificPropertiesToProto(proto::LayerProperties* proto,
                    base->mutable_offset_to_transform_parent());
   base->set_draws_content(draws_content_);
   base->set_may_contain_video(may_contain_video_);
+  // TODO(bokan): Should we serialize the scrollbars_enabled flag?
   base->set_hide_layer_and_subtree(inputs_.hide_layer_and_subtree);
   base->set_subtree_property_changed(subtree_property_changed_);
 
@@ -1454,6 +1457,7 @@ void Layer::FromLayerSpecificPropertiesProto(
   inputs_.double_sided = base.double_sided();
   draws_content_ = base.draws_content();
   may_contain_video_ = base.may_contain_video();
+  // TODO(bokan): scrollbars_hidden_
   inputs_.hide_layer_and_subtree = base.hide_layer_and_subtree();
   subtree_property_changed_ = base.subtree_property_changed();
   inputs_.masks_to_bounds = base.masks_to_bounds();
@@ -1607,6 +1611,15 @@ void Layer::SetMayContainVideo(bool yes) {
   SetNeedsPushProperties();
 }
 
+void Layer::SetScrollbarsHiddenFromImplSide(bool hidden) {
+  if (scrollbars_hidden_ == hidden)
+    return;
+
+  scrollbars_hidden_ = hidden;
+  if (inputs_.client)
+    inputs_.client->didChangeScrollbarsHidden(hidden);
+}
+
 bool Layer::FilterIsAnimating() const {
   return GetAnimationHost()->IsAnimatingFilterProperty(
       element_id(), GetElementTypeForAnimation());