Use completion_time to call LoadingFinished of DevTools's Network event for navigation preload

Currently NetworkHandler::NavigationPreloadCompleted() is using TimeTicks::Now()
to call LoadingFinished() of DevTools's Network event. But for normal requests
from the renderer, FrameFetchContext::DispatchDidFinishLoading() is using the
|finish_time| which is originated from ResourceRequestCompletionStatus's
completion_time.

NetworkHandler::NavigationPreloadCompleted() should use the same timing
information.

I found this difference while investigating the issue 712809. |completion_time|
is set when the request finished but, NavigationPreloadCompleted() isn't called
before the service worker finishes starting up.

BUG=712809

Review-Url: https://codereview.chromium.org/2844133005
Cr-Commit-Position: refs/heads/master@{#467896}

content/browser/devtools/protocol/network_handler.cc

@@ -704,7 +704,8 @@ void NetworkHandler::NavigationPreloadCompleted(
         completion_status.error_code == net::Error::ERR_ABORTED);
   }
   frontend_->LoadingFinished(
-      request_id, base::TimeTicks::Now().ToInternalValue() /
+      request_id,
+      completion_status.completion_time.ToInternalValue() /
           static_cast<double>(base::Time::kMicrosecondsPerSecond),
       completion_status.encoded_data_length);
 }