Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

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 bfb276a0f8ae4e2eb7fd6d3b60ab90b18e33245c..daaccca36ccc062375cb50561a3821b9f8abaacf 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -541,6 +541,23 @@ void LayerTreeImpl::PushPropertiesTo(LayerTreeImpl* target_tree) {
     target_tree->set_hud_layer(NULL);
 
   target_tree->has_ever_been_drawn_ = false;
+
+  // Note: this needs to happen after SetPropertyTrees.
+  target_tree->ShowScrollbars();

[bokan, 2017/03/27 16:25:13]

I think this relies on there always being a pending tree, which I think is the
case on Android but not necessarily all platforms (though maybe my understanding
is outdated here). Maybe this should be in LTHI::ActivateSyncTree?

[skobes, 2017/03/27 19:00:31]

Based on LTHI::CommitToActiveTree it looks like we create a pending tree when
threaded compositing is on, which I believe is everywhere except layout tests. 
Layout tests don't seem to create a ScrollbarAnimationController, so we should
be ok there.

+}
+
+void LayerTreeImpl::ShowScrollbars() {
+  LayerTreeHostCommon::CallFunctionForEveryLayer(this, [this](
+                                                           LayerImpl* layer) {
+    if (!layer->needs_show_scrollbars())
+      return;
+    ScrollbarAnimationController* controller =
+        layer_tree_host_impl_->ScrollbarAnimationControllerForId(layer->id());
+    if (controller) {
+      controller->DidRequestShowFromMainThread();
+      layer->set_needs_show_scrollbars(false);
+    }
+  });
 }
 
 void LayerTreeImpl::MoveChangeTrackingToLayers() {