Reland main thread position sticky implementation.

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 1f384ebfcee48d60cee17f942d8d3128b5c1e632..ea94631c81483cf09de3a23ee298093082a469b7 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -19,6 +19,7 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kScrollbarScrolling = 1 << 3 };
   enum : uint32_t { kPageOverlay = 1 << 4 };
   enum : uint32_t { kAnimatingScrollOnMainThread = 1 << 13 };
+  enum : uint32_t { kHasStickyPositionObjects = 1 << 14 };
 
   // Transient scrolling reasons. These are computed for each scroll begin.
   enum : uint32_t { kNonFastScrollableRegion = 1 << 5 };
@@ -31,7 +32,7 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kPageBasedScrolling = 1 << 12 };
 
   // The number of flags in this struct (excluding itself).
-  enum : uint32_t { kMainThreadScrollingReasonCount = 15 };
+  enum : uint32_t { kMainThreadScrollingReasonCount = 16 };
 
   // Returns true if the given MainThreadScrollingReason can be set by the main
   // thread.
@@ -39,7 +40,8 @@ struct MainThreadScrollingReason {
     uint32_t reasons_set_by_main_thread =
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
-        kScrollbarScrolling | kPageOverlay | kAnimatingScrollOnMainThread;
+        kScrollbarScrolling | kPageOverlay | kAnimatingScrollOnMainThread |
+        kHasStickyPositionObjects;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }