Add main thread scrolling reasons to GraphicsLayer::layerTreeAsJSON.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index e36d7456dfc3b9422d2c54d5677184adea0bff59..e0e09b5d630c48532175ed6f7c0177344743c140 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -870,6 +870,7 @@ void Layer::AddMainThreadScrollingReasons(
   if (inputs_.main_thread_scrolling_reasons == new_reasons)
     return;
   inputs_.main_thread_scrolling_reasons = new_reasons;
+  didUpdateMainThreadScrollingReasons();
   SetNeedsCommit();
 }
 
@@ -882,6 +883,7 @@ void Layer::ClearMainThreadScrollingReasons(
   if (new_reasons == inputs_.main_thread_scrolling_reasons)
     return;
   inputs_.main_thread_scrolling_reasons = new_reasons;
+  didUpdateMainThreadScrollingReasons();
   SetNeedsCommit();
 }
 
@@ -1579,6 +1581,11 @@ Layer::TakeDebugInfo() {
     return nullptr;
 }
 
+void Layer::didUpdateMainThreadScrollingReasons() {
+  if (inputs_.client)
+    inputs_.client->didUpdateMainThreadScrollingReasons();
+}
+
 void Layer::SetSubtreePropertyChanged() {
   if (subtree_property_changed_)
     return;