Fix WebURLLoaderMockFactoryImpl::RunUntilIdle

This can lead to nested messageloops and it's working by accident
currently due to the interactions with the way the SequenceManager and
the MessageLoop work.

Bug: 863341, 891670
Change-Id: I7b01059638e2ba8a136a7daac387467aab517a3c
Reviewed-on: https://chromium-review.googlesource.com/c/1347353Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610394}

third_party/blink/renderer/platform/testing/weburl_loader_mock_factory_impl.cc

@@ -168,10 +168,11 @@ void WebURLLoaderMockFactoryImpl::LoadAsynchronouly(
 }
 
 void WebURLLoaderMockFactoryImpl::RunUntilIdle() {
-  if (platform_)
+  if (platform_) {
     platform_->RunUntilIdle();
-  else
-    base::RunLoop().RunUntilIdle();
+  } else {
+    base::RunLoop(base::RunLoop::Type::kNestableTasksAllowed).RunUntilIdle();
+  }
 }
 
 void WebURLLoaderMockFactoryImpl::LoadRequest(