Wait for QuicServer to process all requests before checking for errors.

The test referred to in the bug below has been flaky on multiple
platforms: iOS(1032568) and FuchsiaOS(813631) as well as Win 7 and
MacOS on Edge. This PR includes a short change which should fix this.

It seems like the QuicServer running asynchronously hasn't had time to
process all requests which is causing a race condition when the check
in line 421 is made. So we wait until the server is finished processing
the requests before making the expect check.

Bug: 1032568
Change-Id: I52dc909b7167d811126fab99cf2b7ad334b2d96e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2008627Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736150}

net/url_request/url_request_quic_unittest.cc

@@ -417,6 +417,10 @@ TEST_P(URLRequestQuicTest, CancelPushIfCached_SomeCached) {
 #if !defined(OS_FUCHSIA) && !defined(OS_IOS)
   // TODO(crbug.com/813631): Make this work on Fuchsia.
   // TODO(crbug.com/1032568): Make this work on iOS.
+
+  // Wait until the server has processed all errors which is
+  // happening asynchronously
+  base::RunLoop().RunUntilIdle();
   // Verify the reset error count received on the server side.
   EXPECT_LE(1u, GetRstErrorCountReceivedByServer(quic::QUIC_STREAM_CANCELLED));
 #endif