New TimeToFirstURLJob* navigation metrics now work with PlzNavigate.

content/browser/frame_host/navigator.h

Index: content/browser/frame_host/navigator.h
diff --git a/content/browser/frame_host/navigator.h b/content/browser/frame_host/navigator.h
index 191a58366227634b7e51db019cc8cdf6f5453019..e56335ad5a77458cfb06b1407eb362149370cf10 100644
--- a/content/browser/frame_host/navigator.h
+++ b/content/browser/frame_host/navigator.h
@@ -138,8 +138,12 @@ class CONTENT_EXPORT Navigator : public base::RefCounted<Navigator> {
   // |frame_tree_node| is destroyed.
   virtual void CancelNavigation(FrameTreeNode* frame_tree_node) {}
 
-  // Called when the first resource request for a given navigation is executed
-  // so that it can be tracked into an histogram.
+  // Called when the network stack started handling the navigation request
+  // so that the |timestamp| when it happened can be recorded into an histogram.
+  // The |url| is used to verify we're tracking the correct navigation.
+  // TODO(carlosk): once PlzNavigate is the only navigation implementation
+  // remove the URL parameter and rename this method to better suit its naming
+  // conventions.
   virtual void LogResourceRequestTime(
     base::TimeTicks timestamp, const GURL& url) {};