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

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 3a64da20ff27cc8540beb7a5f16fa4fe72774714..20161f39fc8c9278990600bbb224cbb67e9e34bb 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -344,8 +344,7 @@ void ScrollingCoordinator::scrollableAreaScrollbarLayerDidChange(ScrollableArea*
 
     if (!platformSupportsCoordinatedScrollbar) {
         if (scrollbarGraphicsLayer) {
-            WebLayer* scrollbarLayer = toWebLayer(scrollbarGraphicsLayer);
-            scrollbarLayer->addMainThreadScrollingReasons(MainThreadScrollingReason::kScrollbarScrolling);
+            scrollbarGraphicsLayer->setMainThreadScrollingReasons(MainThreadScrollingReason::kScrollbarScrolling);

[ajuma, 2016/02/04 15:05:48]

Is changing this from adding a reason to replacing the existing reasons
intentional?

         }
         return;
     }
@@ -684,12 +683,9 @@ void ScrollingCoordinator::setShouldUpdateScrollLayerPositionOnMainThread(MainTh
 {
     if (!m_page->mainFrame()->isLocalFrame() || !m_page->deprecatedLocalMainFrame()->view())
         return;
-    if (WebLayer* scrollLayer = toWebLayer(m_page->deprecatedLocalMainFrame()->view()->layerForScrolling())) {
+    if (GraphicsLayer* scrollLayer = m_page->deprecatedLocalMainFrame()->view()->layerForScrolling()) {
         m_lastMainThreadScrollingReasons = mainThreadScrollingReasons;
-        if (mainThreadScrollingReasons)
-            scrollLayer->addMainThreadScrollingReasons(mainThreadScrollingReasons);
-        else
-            scrollLayer->clearMainThreadScrollingReasons();
+        scrollLayer->setMainThreadScrollingReasons(mainThreadScrollingReasons);
     }
 }