Make statechange test wait for SLD completion

This change makes sure the SetLocalDescription completes
before we call SetRemoteDescription.

If we don't wait, we will sometimes see a change to
have-remote-offer rather than the expected change to
first have-local-offer, then stable (and then
have-remote-offer, but the test doesn't wait for that).

Bug: chromium:1071329
Change-Id: I7eaa06a71edff92e76448ac2ef1fdfecee795ebe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152446
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759615}

third_party/blink/web_tests/TestExpectations

@@ -6588,9 +6588,6 @@ crbug.com/1070995 external/wpt/html/infrastructure/safe-passing-of-structured-da
 
 crbug.com/1057020 [ Fuchsia ] http/tests/streams/chromium/get-reader-byob.html [ Pass Timeout ]
 
-# Ecosystem-Infra Sheriff 2020-04-16
-crbug.com/1071329 fast/peerconnection/statechange-iframe-destroy-parent.html [ Pass Timeout ]
-
 # Sheriff 2020-03-05
 crbug.com/1058073 [ Retina ] virtual/threaded/fast/scroll-snap/snap-to-target-on-layout-change.html [ Pass Failure ]
 crbug.com/1058073 [ Mac10.14 ] accessibility/content-changed-notification-causes-crash.html [ Pass Failure ]

third_party/blink/web_tests/fast/peerconnection/resources/statechange-iframe-destroy-child.html

@@ -18,8 +18,8 @@ function causeIssue() {
   };
 
   youConnection.createOffer(offerOptions)
-    .then(function(offer){
-      youConnection.setLocalDescription(offer);
+    .then(async function(offer) {
+      await youConnection.setLocalDescription(offer);
       // Cause an implicit rollback.
       youConnection.setRemoteDescription(offer);
     });