cc: Move SyncedScrollOffset to scroll tree

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 <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 3231 matching lines @@
   // scales that we want when we're not inside a pinch.
   active_tree_->set_needs_update_draw_properties();
   SetNeedsRedraw();
 }
 
 static void CollectScrollDeltas(ScrollAndScaleSet* scroll_info,
                                 LayerImpl* root_layer) {
   if (!root_layer)
     return;
 
-  for (auto* layer : *root_layer->layer_tree_impl()) {
-    gfx::ScrollOffset scroll_delta = layer->PullDeltaForMainThread();
-
-    if (!scroll_delta.IsZero()) {
-      LayerTreeHostCommon::ScrollUpdateInfo scroll;
-      scroll.layer_id = layer->id();
-      scroll.scroll_delta = gfx::Vector2d(scroll_delta.x(), scroll_delta.y());
-      scroll_info->scrolls.push_back(scroll);
-    }
-  }
+  return root_layer->layer_tree_impl()
+      ->property_trees()
+      ->scroll_tree.CollectScrollDeltas(scroll_info);
 }
 
 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_factor()->PullDeltaForMainThread();
   scroll_info->top_controls_delta =
       active_tree()->top_controls_shown_ratio()->PullDeltaForMainThread();
@@ skipping 698 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc