New TimeToFirstURLJob* navigation metrics now work with PlzNavigate.

content/browser/frame_host/navigation_request.h

Index: content/browser/frame_host/navigation_request.h
diff --git a/content/browser/frame_host/navigation_request.h b/content/browser/frame_host/navigation_request.h
index 649e62885c4775bb9b62e6cf8cae7ee0822abda0..98121628020a6f476b3409ff66fce0c59fb39b3c 100644
--- a/content/browser/frame_host/navigation_request.h
+++ b/content/browser/frame_host/navigation_request.h
@@ -110,6 +110,7 @@ class CONTENT_EXPORT NavigationRequest : public NavigationURLLoaderDelegate {
   void OnResponseStarted(const scoped_refptr<ResourceResponse>& response,
                          scoped_ptr<StreamHandle> body) override;
   void OnRequestFailed(int net_error) override;
+  void AboutToBeginNavigation(base::TimeTicks timestamp) override;

[clamy, 2015/01/27 12:35:03]

How about NavigationStarted? This is called after the navigation begun so
AboutToBeginNavigation seems a bit weird.

[carlosk, 2015/01/27 17:12:03]

Agreed but I don't think NavigationStarted is much better... 

I based my choice on the method names from ResourceDispatcherHostImpl but
something closer to what we actually intend to get from it should be  better.

So, how about AboutToAttemptFirstNetworkRequest?

[clamy, 2015/01/28 13:44:21]

I don't like AboutToAttemptFirstNetworkRequest much. First the AboutTo part:
this function is called asynchronously, so by the time it executes chances are
that we started doing what we were about to do. Second we are sending the
request to the network stack, but we may not be doing an actual network request
if it is cached. And it's fairly long :).

I suggested NavigationStarted because NavigationRequest goes through the
following states:
- NOT_STARTED
- WAITING_FOR_RENDERER_RESPONSE
- STARTED
- RESPONSE_STARTED.

It goes into state STARTED after BeginNavigation has been called (so calling
BeginNavigation means the navigation started).

So I think that something like NavigationStarted, NavigationBegun or
BeginNavigationComplete would be better, since it is about the RDH informing
NavigationRequest that it did what was asked of it (BeginNavigation).

[carlosk, 2015/01/28 15:21:08]

I agree with your point on the AboutTo not being precise as the navigation is
not waiting on this message handling so in fact it might be already under way
when the message is handled. But I don't agree it should relate ti the named
states of NavigationRequest (NR) as the information it conveys regards solely
the NavigationURLLoader. The NR is merely observing through the
NavigationURLLoaderDelegate interface and its state change to STARTED is has no
relation to this notification.

Having this said, BeginNavigationComplete is a good option but I also suggest
BeginNavigationRequested or NavigationRequested. I'm preemptively changing to
NavigationRequested that I find short and clear but please let me know WDYT.

 
   FrameTreeNode* frame_tree_node_;