Rename kAnimatingScrollOnMainThread to kHandlingScrollFromMainThread

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 0a1c1da4ce0b8ac509a7fd0736c32fe14f79e680..5ba5792e63f7f8fc937868a9b22a0a7fd3775c91 100644
--- a/cc/input/main_thread_scrolling_reason.h
+++ b/cc/input/main_thread_scrolling_reason.h
@@ -22,7 +22,9 @@ struct MainThreadScrollingReason {
   enum : uint32_t { kThreadedScrollingDisabled = 1 << 2 };
   enum : uint32_t { kScrollbarScrolling = 1 << 3 };
   enum : uint32_t { kPageOverlay = 1 << 4 };
-  enum : uint32_t { kAnimatingScrollOnMainThread = 1 << 13 };
+  // A scroll handled by the main thread can result in an animation running on
+  // the main thread or on the compositor thread.

[skobes, 2016/09/01 04:00:18]

This comment is a great explanation for why you are renaming the bit, but reads
pretty confusingly as documentation of "what does this bit actually mean". :)  
I'd suggest adding a little more context while you're in here about how and why
we set this.

[ymalik, 2016/09/01 14:24:29]

You're absolutely right. Fixed.

+  enum : uint32_t { kHandlingScrollFromMainThread = 1 << 13 };
   enum : uint32_t { kHasStickyPositionObjects = 1 << 14 };
   enum : uint32_t { kCustomScrollbarScrolling = 1 << 15 };
 
@@ -45,7 +47,7 @@ struct MainThreadScrollingReason {
     uint32_t reasons_set_by_main_thread =
         kNotScrollingOnMain | kHasBackgroundAttachmentFixedObjects |
         kHasNonLayerViewportConstrainedObjects | kThreadedScrollingDisabled |
-        kScrollbarScrolling | kPageOverlay | kAnimatingScrollOnMainThread |
+        kScrollbarScrolling | kPageOverlay | kHandlingScrollFromMainThread |
         kHasStickyPositionObjects | kCustomScrollbarScrolling;
     return (reasons & reasons_set_by_main_thread) == reasons;
   }
@@ -87,8 +89,8 @@ struct MainThreadScrollingReason {
       tracedValue->AppendString("Scrollbar scrolling");
     if (reasons & MainThreadScrollingReason::kPageOverlay)
       tracedValue->AppendString("Page overlay");
-    if (reasons & MainThreadScrollingReason::kAnimatingScrollOnMainThread)
-      tracedValue->AppendString("Animating scroll on main thread");
+    if (reasons & MainThreadScrollingReason::kHandlingScrollFromMainThread)
+      tracedValue->AppendString("Handling scroll from main thread");
     if (reasons & MainThreadScrollingReason::kHasStickyPositionObjects)
       tracedValue->AppendString("Has sticky position objects");
     if (reasons & MainThreadScrollingReason::kCustomScrollbarScrolling)