Don't set sent_top_controls_delta when root layer doesn't exist.

cc/trees/layer_tree_host_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_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 2886 matching lines @@
     pinch_gesture_end_should_clear_scrolling_layer_ = false;
     ClearCurrentlyScrollingLayer();
   }
   if (top_controls_manager_)
     top_controls_manager_->PinchEnd();
   client_->SetNeedsCommitOnImplThread();
 }
 
 static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
                                 LayerImpl* layer_impl) {
-  if (!layer_impl)
-    return;
+  DCHECK(layer_impl);
 
   gfx::Vector2d scroll_delta =
       gfx::ToFlooredVector2d(layer_impl->ScrollDelta());
   if (!scroll_delta.IsZero()) {
     LayerTreeHostCommon::ScrollUpdateInfo scroll;
     scroll.layer_id = layer_impl->id();
     scroll.scroll_delta = scroll_delta;
     scroll_info->scrolls.push_back(scroll);
     layer_impl->SetSentScrollDelta(scroll_delta);
   }
 
   for (size_t i = 0; i < layer_impl->children().size(); ++i)
     CollectScrollDeltas(scroll_info, layer_impl->children()[i]);
 }
 
 scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   scoped_ptr<ScrollAndScaleSet> scroll_info(new ScrollAndScaleSet());
 
-  CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
-  scroll_info->page_scale_delta = active_tree_->page_scale_delta();
-  active_tree_->set_sent_page_scale_delta(scroll_info->page_scale_delta);
-  scroll_info->swap_promises.swap(swap_promises_for_main_thread_scroll_update_);
-  scroll_info->top_controls_delta = active_tree()->top_controls_delta();
-  active_tree_->set_sent_top_controls_delta(scroll_info->top_controls_delta);
+  if (active_tree_->root_layer()) {
+    CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
+    scroll_info->page_scale_delta = active_tree_->page_scale_delta();
+    active_tree_->set_sent_page_scale_delta(scroll_info->page_scale_delta);
+    scroll_info->swap_promises.swap(
+        swap_promises_for_main_thread_scroll_update_);
+    scroll_info->top_controls_delta = active_tree()->top_controls_delta();
+    active_tree_->set_sent_top_controls_delta(scroll_info->top_controls_delta);
+  }
 
   return scroll_info.Pass();
 }
 
 void LayerTreeHostImpl::SetFullRootLayerDamage() {
   SetViewportDamage(gfx::Rect(DrawViewportSize()));
 }
 
 void LayerTreeHostImpl::ScrollViewportInnerFirst(gfx::Vector2dF scroll_delta) {
   DCHECK(InnerViewportScrollLayer());
@@ skipping 442 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc