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

cc/blink/web_layer_impl.cc

 // Copyright 2014 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/blink/web_layer_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <utility>
@@ skipping 302 matching lines @@
 void WebLayerImpl::addMainThreadScrollingReasons(
     uint32_t main_thread_scrolling_reasons) {
   // WebLayerImpl should only know about non-transient scrolling
   // reasons. Transient scrolling reasons are computed per hit test.
   DCHECK(main_thread_scrolling_reasons);
   DCHECK(cc::MainThreadScrollingReason::MainThreadCanSetScrollReasons(
       main_thread_scrolling_reasons));
   layer_->AddMainThreadScrollingReasons(main_thread_scrolling_reasons);
 }
 
-void WebLayerImpl::clearMainThreadScrollingReasons() {
-  layer_->ClearMainThreadScrollingReasons();
+void WebLayerImpl::clearMainThreadScrollingReasons(
+    uint32_t main_thread_scrolling_reasons_to_clear) {
+  layer_->ClearMainThreadScrollingReasons(
+      main_thread_scrolling_reasons_to_clear);
+}
+
+uint32_t WebLayerImpl::mainThreadScrollingReasons() {
+  return layer_->main_thread_scrolling_reasons();
 }
 
 bool WebLayerImpl::shouldScrollOnMainThread() const {
   return layer_->should_scroll_on_main_thread();
 }
 
 void WebLayerImpl::setNonFastScrollableRegion(const WebVector<WebRect>& rects) {
   cc::Region region;
   for (size_t i = 0; i < rects.size(); ++i)
     region.Union(rects[i]);
@@ skipping 153 matching lines @@
 
 Layer* WebLayerImpl::layer() const {
   return layer_.get();
 }
 
 void WebLayerImpl::SetContentsOpaqueIsFixed(bool fixed) {
   contents_opaque_is_fixed_ = fixed;
 }
 
 }  // namespace cc_blink