Add main thread scrolling reasons to GraphicsLayer::layerTreeAsJSON.

third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp

Index: third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
index 6b219ce45259d345e90cdd6a2b445c4088a1bcc5..d4f63e7e2a0d4d1ab2f35d42064d46a822496d48 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
@@ -385,6 +385,8 @@ void ScrollAnimator::addMainThreadScrollingReason()
     if (WebLayer* scrollLayer = toWebLayer(getScrollableArea()->layerForScrolling())) {
         scrollLayer->addMainThreadScrollingReasons(
             MainThreadScrollingReason::kAnimatingScrollOnMainThread);
+        getScrollableArea()->layerForScrolling()->debugInfo().setMainThreadScrollingReasons(
+            scrollLayer->mainThreadScrollingReasons());
     }
 }
 
@@ -393,6 +395,8 @@ void ScrollAnimator::removeMainThreadScrollingReason()
     if (WebLayer* scrollLayer = toWebLayer(getScrollableArea()->layerForScrolling())) {
         scrollLayer->clearMainThreadScrollingReasons(
             MainThreadScrollingReason::kAnimatingScrollOnMainThread);
+        getScrollableArea()->layerForScrolling()->debugInfo().setMainThreadScrollingReasons(
+            scrollLayer->mainThreadScrollingReasons());
     }
 }