cc: Make custom scrollbar scroll on main thread

cc/input/main_thread_scrolling_reason.h

Index: cc/input/main_thread_scrolling_reason.h
diff --git a/cc/input/main_thread_scrolling_reason.h b/cc/input/main_thread_scrolling_reason.h
index ea94631c81483cf09de3a23ee298093082a469b7..522909afb1eaed1b959ee19f6abce3a53d13f053 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -20,6 +20,7 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kPageOverlay = 1 << 4 };
   enum : uint32_t { kAnimatingScrollOnMainThread = 1 << 13 };
   enum : uint32_t { kHasStickyPositionObjects = 1 << 14 };
+  enum : uint32_t { kCustomScrollbarScrolling = 1 << 15 };
 
   // Transient scrolling reasons. These are computed for each scroll begin.
   enum : uint32_t { kNonFastScrollableRegion = 1 << 5 };
@@ -32,7 +33,7 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kPageBasedScrolling = 1 << 12 };
 
   // The number of flags in this struct (excluding itself).
-  enum : uint32_t { kMainThreadScrollingReasonCount = 16 };
+  enum : uint32_t { kMainThreadScrollingReasonCount = 17 };
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
@@ -41,7 +42,7 @@ struct MainThreadScrollingReason {
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
         kScrollbarScrolling | kPageOverlay | kAnimatingScrollOnMainThread |
-        kHasStickyPositionObjects;
+        kHasStickyPositionObjects | kCustomScrollbarScrolling;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }