Fix smooth scroll jump when switching scroll handling between MT and CC

cc/layers/layer.cc

 // Copyright 2010 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/layers/layer.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 942 matching lines @@
 void Layer::AddMainThreadScrollingReasons(
     uint32_t main_thread_scrolling_reasons) {
   DCHECK(IsPropertyChangeAllowed());
   DCHECK(main_thread_scrolling_reasons);
   if (main_thread_scrolling_reasons_ == main_thread_scrolling_reasons)
     return;
   main_thread_scrolling_reasons_ |= main_thread_scrolling_reasons;
   SetNeedsCommit();
 }
 
+void Layer::ClearMainThreadScrollingReasons(
+    uint32_t main_thread_scrolling_reasons_to_clear) {
+  DCHECK(IsPropertyChangeAllowed());
+  DCHECK(main_thread_scrolling_reasons_to_clear);
+  main_thread_scrolling_reasons_ =
+      ~main_thread_scrolling_reasons_to_clear & main_thread_scrolling_reasons_;
+  SetNeedsCommit();
+}
+
 void Layer::ClearMainThreadScrollingReasons() {
   DCHECK(IsPropertyChangeAllowed());
   if (!main_thread_scrolling_reasons_)
     return;
   main_thread_scrolling_reasons_ =
       MainThreadScrollingReason::kNotScrollingOnMain;
   SetNeedsCommit();
 }
 
 void Layer::SetNonFastScrollableRegion(const Region& region) {
@@ skipping 1038 matching lines @@
       this, layer_tree_host_->property_trees()->transform_tree);
 }
 
 gfx::Transform Layer::screen_space_transform() const {
   DCHECK_NE(transform_tree_index_, -1);
   return ScreenSpaceTransformFromPropertyTrees(
       this, layer_tree_host_->property_trees()->transform_tree);
 }
 
 }  // namespace cc