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 8e05fde637a8f3da293befada619be84582149eb..f9935f5bb7d3031486bb06c4402c7a7382e73e7c 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3570,8 +3570,9 @@ 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",