Prevent tracking metrics for cases such as 204s and downloads.

content/public/browser/navigation_handle.h

Index: content/public/browser/navigation_handle.h
diff --git a/content/public/browser/navigation_handle.h b/content/public/browser/navigation_handle.h
index f01f0748dd670f44cfcce4e25f46fb0e43a80acd..8770777171da92bf15cb92d1d4a0117004c22115 100644
--- a/content/public/browser/navigation_handle.h
+++ b/content/public/browser/navigation_handle.h
@@ -159,10 +159,11 @@ class CONTENT_EXPORT NavigationHandle {
   // GetNetErrorCode will be net::OK.
   virtual bool IsErrorPage() = 0;
 
-  // Returns the response headers for the request or nullptr if there are none.
-  // This should only be accessed after a redirect was encountered or after the
-  // navigation is ready to commit. The headers returned should not be modified,
-  // as modifications will not be reflected in the network stack.
+  // Returns the response headers for the request, or nullptr if there aren't
+  // any response headers or they have not been received yet. The response
+  // headers may change during the navigation (e.g. after encountering a server
+  // redirect). The headers returned should not be modified, as modifications
+  // will not be reflected in the network stack.
   virtual const net::HttpResponseHeaders* GetResponseHeaders() = 0;
 
   // Resumes a navigation that was previously deferred by a NavigationThrottle.