Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

cc/trees/layer_tree_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 523 matching lines @@
   else
     target_tree->ResetViewportSizeInvalid();
 
   if (hud_layer())
     target_tree->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(
         target_tree->LayerById(hud_layer()->id())));
   else
     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is outdated here).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() {
   // We need to update the change tracking on property trees before we move it
   // onto the layers.
   property_trees_.UpdateChangeTracking();
   for (auto* layer : *this) {
     if (layer->LayerPropertyChanged())
       layer->NoteLayerPropertyChanged();
   }
@@ skipping 1603 matching lines @@
 
 void LayerTreeImpl::ResetAllChangeTracking() {
   layers_that_should_push_properties_.clear();
   // Iterate over all layers, including masks.
   for (auto& layer : *layers_)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking();
 }
 
 }  // namespace cc