cc: Make custom scrollbar scroll on main thread

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 ee693987ba58228eafbed3919ce9910543964adb..fa9ba97b8d10b101775c89656d6006bd25d5b489 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -343,6 +343,7 @@ void ScrollingCoordinator::scrollableAreaScrollbarLayerDidChange(ScrollableArea*
         Scrollbar& scrollbar = orientation == HorizontalScrollbar ? *scrollableArea->horizontalScrollbar() : *scrollableArea->verticalScrollbar();
         if (scrollbar.isCustomScrollbar()) {
             detachScrollbarLayer(scrollbarGraphicsLayer);
+            scrollbarGraphicsLayer->platformLayer()->addMainThreadScrollingReasons(MainThreadScrollingReason::kCustomScrollbarScrolling);

[ajuma, 2016/06/22 14:41:41]

I think you also need an 'else' case to remove the main-thread scrolling reason
(to handle the case of a scrollbar that switches from custom to non-custom).

             return;
         }