Add a |scope| argument to TRACE_EVENT_INSTANT* and require its presence.

cc/layer_tree_host.cc

Index: cc/layer_tree_host.cc
diff --git a/cc/layer_tree_host.cc b/cc/layer_tree_host.cc
index 531eab6e8d881d39896443e1291f641e40139440..12d1f518e332537a583f177d7666ffd7ef6d1d20 100644
--- a/cc/layer_tree_host.cc
+++ b/cc/layer_tree_host.cc
@@ -425,7 +425,8 @@ void LayerTreeHost::setNeedsAnimate()
 void LayerTreeHost::setNeedsCommit()
 {
     if (!m_prepaintCallback.IsCancelled()) {
-        TRACE_EVENT_INSTANT0("cc", "LayerTreeHost::setNeedsCommit::cancel prepaint");
+        TRACE_EVENT_INSTANT0("cc", "LayerTreeHost::setNeedsCommit::cancel prepaint",
+                             TRACE_EVENT_SCOPE_THREAD);
         m_prepaintCallback.Cancel();
     }
     m_proxy->setNeedsCommit();