Ignore "Synchronous XHR in page dismissal" error thrown by AppRTC

https://github.com/webrtc/apprtc/blob/20cdd7652d58c9cf47ef92ba0190a5505760dc05/src/web_app/js/call.js#L159
sends an XHR on page close which is now an error in Chromium. Ignoring the error doesn't cause any problems, especially for the purpose of this test, so until a real fix is made, this is a simple way to un-break tests.
(https://ci.chromium.org/p/chromium/g/chromium.webrtc)

Bug: 918871
Change-Id: I29497a39424a1c54e7b6875edc0cb896663221ea
Reviewed-on: https://chromium-review.googlesource.com/c/1396019Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619943}

chrome/browser/media/webrtc/webrtc_browsertest_base.cc

@@ -89,6 +89,10 @@ bool JavascriptErrorDetectingLogHandler(int severity,
   if (file == NULL || std::string("CONSOLE") != file)
     return false;
 
+  // TODO(crbug.com/918871): Fix AppRTC and stop ignoring this error.
+  if (str.find("Synchronous XHR in page dismissal") != std::string::npos)
+    return false;
+
   bool contains_uncaught = str.find("\"Uncaught ") != std::string::npos;
   if (severity == logging::LOG_ERROR ||
       (severity == logging::LOG_INFO && contains_uncaught)) {