Add tracing output for main thread scroll reason

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I5994c5a3fc087c80441009ac1316752f1cfafff6
Reviewed-on: https://chromium-review.googlesource.com/964719Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543808}

cc/trees/layer_tree_host_impl.cc

@@ -2939,8 +2939,10 @@ InputHandler::ScrollStatus LayerTreeHostImpl::TryScroll(
   InputHandler::ScrollStatus scroll_status;
   scroll_status.main_thread_scrolling_reasons =
       MainThreadScrollingReason::kNotScrollingOnMain;
-  if (!!scroll_node->main_thread_scrolling_reasons) {
-    TRACE_EVENT0("cc", "LayerImpl::TryScroll: Failed ShouldScrollOnMainThread");
+  if (scroll_node->main_thread_scrolling_reasons) {
+    TRACE_EVENT1("cc", "LayerImpl::TryScroll: Failed ShouldScrollOnMainThread",
+                 "MainThreadScrollingReason",
+                 scroll_node->main_thread_scrolling_reasons);
     scroll_status.thread = InputHandler::SCROLL_ON_MAIN_THREAD;
     scroll_status.main_thread_scrolling_reasons =
         scroll_node->main_thread_scrolling_reasons;