cc: Move SyncedScrollOffset to scroll tree

cc/input/scroll_state.cc

 // Copyright 2015 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/input/scroll_state.h"
 
 #include <utility>
 
 #include "cc/trees/layer_tree_impl.h"
 
 namespace cc {
 
-ScrollState::ScrollState(ScrollStateData data) : data_(data) {}
+ScrollState::ScrollState(ScrollStateData data)
+    : data_(data), layer_tree_impl_(nullptr) {}
 
 ScrollState::ScrollState(const ScrollState& other) = default;
 
 ScrollState::~ScrollState() {}
 
 void ScrollState::ConsumeDelta(double x, double y) {
   data_.delta_x -= x;
   data_.delta_y -= y;
 
   if (x || y)
     data_.delta_consumed_for_scroll_sequence = true;
 }
 
 void ScrollState::DistributeToScrollChainDescendant() {
   if (!scroll_chain_.empty()) {
     const ScrollNode* next = scroll_chain_.front();
     scroll_chain_.pop_front();
     layer_tree_impl_->LayerById(next->owner_id)->DistributeScroll(this);
   }
 }
 
 }  // namespace cc