PlzNavigate: add initial traces for new TBM benchmarks.

content/browser/frame_host/navigation_request.cc

Index: content/browser/frame_host/navigation_request.cc
diff --git a/content/browser/frame_host/navigation_request.cc b/content/browser/frame_host/navigation_request.cc
index 33e2560ee6cc008bdcaba4414de3c60355e4b0f2..ef2622bde509aac7d445b4f899982592b289def6 100644
--- a/content/browser/frame_host/navigation_request.cc
+++ b/content/browser/frame_host/navigation_request.cc
@@ -281,6 +281,11 @@ void NavigationRequest::OnRequestFailed(bool has_stale_copy_in_cache,
 }
 
 void NavigationRequest::OnRequestStarted(base::TimeTicks timestamp) {
+  if (frame_tree_node_->IsMainFrame()) {
+    TRACE_EVENT_ASYNC_END_WITH_TIMESTAMP0(
+        "navigation", "Navigation timeToIOStartBeforeThrottles",
+        navigation_handle_.get(), timestamp.ToInternalValue());
+  }
   frame_tree_node_->navigator()->LogResourceRequestTime(timestamp,
                                                         common_params_.url);
 }