Add main thread scrolling reasons to GraphicsLayer::layerTreeAsJSON.

third_party/WebKit/Source/web/PageOverlay.cpp

Index: third_party/WebKit/Source/web/PageOverlay.cpp
diff --git a/third_party/WebKit/Source/web/PageOverlay.cpp b/third_party/WebKit/Source/web/PageOverlay.cpp
index e113b7fedaa56ef73f5595da285f60279eb83e27..8e03a2124a8d04d4a728ca13be6b1bb6644d37f2 100644
--- a/third_party/WebKit/Source/web/PageOverlay.cpp
+++ b/third_party/WebKit/Source/web/PageOverlay.cpp
@@ -89,6 +89,7 @@ void PageOverlay::update()
         // This is required for contents of overlay to stay in sync with the page while scrolling.
         WebLayer* platformLayer = m_layer->platformLayer();
         platformLayer->addMainThreadScrollingReasons(MainThreadScrollingReason::kPageOverlay);
+        m_layer->debugInfo().setMainThreadScrollingReasons(platformLayer->mainThreadScrollingReasons());
         page->frameHost().visualViewport().containerLayer()->addChild(m_layer.get());
     }