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..eb1af688b8aa9a4c64c30353e1363353b172dc8c 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(

[clamy, 2015/12/18 15:42:11]

I'm not sure about the naming of the measurement - start may be a bit vague. For
exemple, do we register the trace when the net stack receives the request or
when the various throttles have executed and the network request actually
starts?

[carlosk, 2015/12/18 17:23:21]

Looking at the code this is triggered from, it happens along with the call to
ResourceDispatcherHostImpl::BeginNavigationRequest which checks with
ResourceHandlers (that go through the throttles as well). So it's before the net
stack actually starts.

It seems this is OK as a point of measurement but I agree we might need to
rename it in this case. I'm thinking "Navigation timeToIOStartBeforeThrottles"
but I'm all ears for better ideas.

Another option is to move this further down so that this is tracked after the
handlers/throttlers. Or even adding another measurement at that point.

[clamy, 2015/12/21 10:43:32]

I'm fine with the measurement happening when it happens. For the name, maybe
timeToNetworkStack (I'm not very fond of it either - hopefully Nasko will have
better ideas).

[carlosk, 2015/12/21 11:19:25]

Done.

+        "navigation", "Navigation timeToIOStart", navigation_handle_.get(),
+        timestamp.ToInternalValue());
+  }
   frame_tree_node_->navigator()->LogResourceRequestTime(timestamp,
                                                         common_params_.url);
 }