blink: Make WebMainThreadScrollingReason wrap the cc enum.

cc/input/main_thread_scrolling_reason.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 #define CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_
 
 namespace cc {
 
-// Ensure this stays in sync with MainThreadScrollingReason in histograms.xml,
-// and that this extends ScrollingCoordinator::MainThreadScrollingReason.
-// ScrollingCoordinator::MainThreadScrollingReason contains the flags
-// which are associated with a layer. The flags only contained in
-// MainThreadScrollingReason are computed for each scroll begin.
+// Ensure this stays in sync with MainThreadScrollingReason in histograms.xml.
 struct MainThreadScrollingReason {
   // Non-transient scrolling reasons.
   enum : uint32_t { kNotScrollingOnMain = 0 };
   enum : uint32_t { kHasBackgroundAttachmentFixedObjects = 1 << 0 };
   enum : uint32_t { kHasNonLayerViewportConstrainedObjects = 1 << 1 };
   enum : uint32_t { kThreadedScrollingDisabled = 1 << 2 };
   enum : uint32_t { kScrollbarScrolling = 1 << 3 };
   enum : uint32_t { kPageOverlay = 1 << 4 };
   enum : uint32_t { kMaxNonTransientScrollingReason = kPageOverlay };
 
-  // Transient scrolling reasons.
+  // Transient scrolling reasons. These are computed for each scroll begin.
   enum : uint32_t { kNonFastScrollableRegion = 1 << 5 };
   enum : uint32_t { kEventHandlers = 1 << 6 };
   enum : uint32_t { kFailedHitTest = 1 << 7 };
   enum : uint32_t { kNoScrollingLayer = 1 << 8 };
   enum : uint32_t { kNotScrollable = 1 << 9 };
   enum : uint32_t { kContinuingMainThreadScroll = 1 << 10 };
   enum : uint32_t { kNonInvertibleTransform = 1 << 11 };
   enum : uint32_t { kPageBasedScrolling = 1 << 12 };
 
   // The number of flags in this struct (excluding itself).
   enum : uint32_t { kMainThreadScrollingReasonCount = 14 };
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_MAIN_THREAD_SCROLLING_REASON_H_