Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 89 matching lines @@
       m_needsCompositedScrolling(false),
       m_rebuildHorizontalScrollbarLayer(false),
       m_rebuildVerticalScrollbarLayer(false),
       m_needsScrollOffsetClamp(false),
       m_needsRelayout(false),
       m_hadHorizontalScrollbarBeforeRelayout(false),
       m_hadVerticalScrollbarBeforeRelayout(false),
       m_scrollbarManager(*this),
       m_scrollCorner(nullptr),
       m_resizer(nullptr),
-      m_scrollAnchor(this)
+      m_scrollAnchor(this),
+      m_reasons(0)
 #if DCHECK_IS_ON()
       ,
       m_hasBeenDisposed(false)
 #endif
 {
   Node* node = box().node();
   if (node && node->isElementNode()) {
     // We save and restore only the scrollOffset as the other scroll values are
     // recalculated.
     Element* element = toElement(node);
@@ skipping 1912 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink