PlzNavigate: add initial traces for new TBM benchmarks.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 39cc6cd72f52e7af29b8b8e2565be3a110554630..d6f788ad7c1e9b37123cadede7fa35a451b77080 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3582,8 +3582,9 @@ void WebContentsImpl::SetIsLoading(bool is_loading,
 
   std::string url = (details ? details->url.possibly_invalid_spec() : "NULL");
   if (is_loading) {
-    TRACE_EVENT_ASYNC_BEGIN1("browser,navigation", "WebContentsImpl Loading",
-                             this, "URL", url);
+    TRACE_EVENT_ASYNC_BEGIN2("browser,navigation", "WebContentsImpl Loading",
+                             this, "URL", url, "Main FrameTreeNode id",
+                             GetFrameTree()->root()->frame_tree_node_id());
     FOR_EACH_OBSERVER(WebContentsObserver, observers_, DidStartLoading());
   } else {
     TRACE_EVENT_ASYNC_END1("browser,navigation", "WebContentsImpl Loading",