Fix RTCRtpReceiver-getContributingSources.https.html flake.

The flake was
"Failed to execute 'addTrack' on 'RTCPeerConnection': The
RTCPeerConnection's signalingState is 'closed'."

The promise_test could be torn down and execute
t.add_cleanup(() => pc1.close());
before connectAndExpectNoCsrcs() was ready due to not using "await" when
calling the test helper method.

TBR=hta@chromium.org
NOTRY=True

// See #14791

Bug: None
Change-Id: I1dac26837b3f29b8f1c38709c7a8488d0e2580c7
Reviewed-on: https://chromium-review.googlesource.com/c/1424891
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624622}

webrtc/RTCRtpReceiver-getContributingSources.https.html

@@ -26,10 +26,10 @@
 }
 
 promise_test(async t => {
-  connectAndExpectNoCsrcs(t, 'audio');
+  await connectAndExpectNoCsrcs(t, 'audio');
 }, '[audio] getContributingSources() returns an empty list in loopback call');
 
 promise_test(async t => {
-  connectAndExpectNoCsrcs(t, 'video');
+  await connectAndExpectNoCsrcs(t, 'video');
 }, '[video] getContributingSources() returns an empty list in loopback call');
 </script>