[GCM] Run InstanceIDImpl callbacks asynchronously

chrome/browser/push_messaging/push_messaging_browsertest.cc

Index: chrome/browser/push_messaging/push_messaging_browsertest.cc
diff --git a/chrome/browser/push_messaging/push_messaging_browsertest.cc b/chrome/browser/push_messaging/push_messaging_browsertest.cc
index b11c28c6b9360b9a0be3e3ff777776947f6fc8b7..6b2cdbf8982683ffa68d709c0ddb78d72673e6d2 100644
--- a/chrome/browser/push_messaging/push_messaging_browsertest.cc
+++ b/chrome/browser/push_messaging/push_messaging_browsertest.cc
@@ -2223,6 +2223,9 @@ IN_PROC_BROWSER_TEST_F(
   histogram_tester_.ExpectUniqueSample(
       "PushMessaging.UnregistrationReason",
       content::PUSH_UNREGISTRATION_REASON_PERMISSION_REVOKED, 1);
+
+  base::RunLoop().RunUntilIdle();
+
   // Revoked permission should trigger an automatic unsubscription attempt.
   EXPECT_EQ(app_id, gcm_driver_->last_deletetoken_app_id());
 }