Releasing Track & Thumb UIResources based on their Geometry

cc/layers/painted_scrollbar_layer.cc

Index: cc/layers/painted_scrollbar_layer.cc
diff --git a/cc/layers/painted_scrollbar_layer.cc b/cc/layers/painted_scrollbar_layer.cc
index 7062bc1e9bade7910cde353e5be9697cd9922e80..65fb78d373bba77174dd9c5087b73396e34629be 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -130,8 +130,12 @@ void PaintedScrollbarLayer::PushPropertiesTo(LayerImpl* layer) {
 
   if (track_resource_.get())
     scrollbar_layer->set_track_ui_resource_id(track_resource_->id());
+  else
+    scrollbar_layer->set_track_ui_resource_id(0);
   if (thumb_resource_.get())
     scrollbar_layer->set_thumb_ui_resource_id(thumb_resource_->id());
+  else
+    scrollbar_layer->set_thumb_ui_resource_id(0);
 
   scrollbar_layer->set_is_overlay_scrollbar(is_overlay_);
 }
@@ -206,8 +210,23 @@ bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
   gfx::Rect scaled_track_rect = ScrollbarLayerRectToContentRect(
       track_layer_rect);
 
-  if (track_rect_.IsEmpty() || scaled_track_rect.IsEmpty())
-    return false;
+  bool updated = false;
+
+  if (track_rect_.IsEmpty() || scaled_track_rect.IsEmpty()) {
+    if (track_resource_) {
+      track_resource_.reset();
+      if (thumb_resource_)
+        thumb_resource_.reset();
+      SetNeedsPushProperties();
+      updated = true;
+    }
+    return updated;
+  }
+
+  if (!has_thumb_ && thumb_resource_) {
+    thumb_resource_.reset();
+    SetNeedsPushProperties();
+  }
 
   {
     base::AutoReset<bool> ignore_set_needs_commit(&ignore_set_needs_commit_,
@@ -216,7 +235,7 @@ bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
   }
 
   if (update_rect_.IsEmpty() && track_resource_)
-    return false;
+    return updated;
 
   track_resource_ = ScopedUIResource::Create(
       layer_tree_host(),
@@ -233,7 +252,8 @@ bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
 
   // UI resources changed so push properties is needed.
   SetNeedsPushProperties();
-  return true;
+  updated = true;
+  return updated;
 }
 
 UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(