Add a debug log upon receiving XMPP notification to check for updates.

The message will help identify whether changelist fetch was triggered
by a push notification or internal polling timer from the logs in
about://drive-internals.

Bug: 783184
Change-Id: I3463aad2751b0adab7d708e0c2eb636893ea550d
Reviewed-on: https://chromium-review.googlesource.com/790037Reviewed-by: Tomasz Mikolajewski <mtomasz@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519582}

chrome/browser/chromeos/drive/drive_integration_service.cc

@@ -403,6 +403,8 @@ void DriveIntegrationService::RemoveObserver(
 }
 
 void DriveIntegrationService::OnNotificationReceived() {
+  logger_->Log(logging::LOG_INFO,
+               "Received Drive update notification. Will check for update.");
   file_system_->CheckForUpdates();
   drive_app_registry_->Update();
 }