Add more specific metrics for main thread scrolling reasons

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

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index e50d441318e02d9e712f2b5845123466b39657b5..3cf4f29dc640cdc3922d67474f070deaf6383a25 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -107,7 +107,8 @@ PaintLayerScrollableArea::PaintLayerScrollableArea(PaintLayer& layer)
       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)
@@ -147,8 +148,18 @@ void PaintLayerScrollableArea::dispose() {
 
   if (box().frame() && box().frame()->page()) {
     if (ScrollingCoordinator* scrollingCoordinator =
-            box().frame()->page()->scrollingCoordinator())
+            box().frame()->page()->scrollingCoordinator()) {
+      // Decrese the number of layer that has any main thread
+      // scrolling reasons stored in ScrollingCoordinator
+      for (uint32_t i = 0;
+           i < MainThreadScrollingReason::kMainThreadScrollingReasonCount;
+           ++i) {
+        if (hasMainThreadScrollingReason(1 << i)) {
+          scrollingCoordinator->adjustMainThreadReasonsFromStyle(1 << i, false);
+        }
+      }
       scrollingCoordinator->willDestroyScrollableArea(this);

[bokan, 2016/12/14 13:28:59]

Actually, since we already have this willDestroyScrollableArea in
ScrollingCoordinator, it makes more sense for the above code to go in there
since its specific to ScrollingCoordinator.

[yigu, 2016/12/14 21:17:33]

Done.

+    }
   }
 
   if (!box().documentBeingDestroyed()) {