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 936 matching lines @@
     return;
   user_scrollable_horizontal_ = horizontal;
   user_scrollable_vertical_ = vertical;
   SetNeedsCommit();
 }
 
 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)
+  uint32_t new_reasons =
+      main_thread_scrolling_reasons_ | main_thread_scrolling_reasons;
+  if (main_thread_scrolling_reasons_ == new_reasons)
     return;
-  main_thread_scrolling_reasons_ |= main_thread_scrolling_reasons;
+  main_thread_scrolling_reasons_ = new_reasons;
   SetNeedsCommit();
 }
 
-void Layer::ClearMainThreadScrollingReasons() {
+void Layer::ClearMainThreadScrollingReasons(
+    uint32_t main_thread_scrolling_reasons_to_clear) {
   DCHECK(IsPropertyChangeAllowed());
-  if (!main_thread_scrolling_reasons_)
+  DCHECK(main_thread_scrolling_reasons_to_clear);
+  uint32_t new_reasons =
+      ~main_thread_scrolling_reasons_to_clear & main_thread_scrolling_reasons_;
+  if (new_reasons == main_thread_scrolling_reasons_)
     return;
-  main_thread_scrolling_reasons_ =
-      MainThreadScrollingReason::kNotScrollingOnMain;
+  main_thread_scrolling_reasons_ = new_reasons;
   SetNeedsCommit();
 }
 
 void Layer::SetNonFastScrollableRegion(const Region& region) {
   DCHECK(IsPropertyChangeAllowed());
   if (non_fast_scrollable_region_ == region)
     return;
   non_fast_scrollable_region_ = region;
   SetNeedsCommit();
 }
@@ skipping 1032 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