Fix swarming tests not running in parallel

Due to recent Chrome infra changes in
https://chromium-review.googlesource.com/c/472290/
tests running on swarming are now assumed to emit JSON results
or will be marked as failing. This requires us to use our
gtest-parallel wrapper for all our Swarming tests
(or implement the --isolated-script-test-output flag, which
normally only is implemented by the Chromium test launcher).

The low_bandwidth_audio_test can actually run in parallel,
so just change that.

The webrtc_nonparallel_tests cannot, so this CL changes MB
to pass --workers=1 flag to gtest-parallel, which makes the
tests run in sequence. This adds a little confusion but the root
problem is really that our gtest-parallel script [1] does a lot more
than just running the tests in parallel these days, so it should
probably be renamed.

Also make sure gtest-parallel-wrapper.py [2] consumes the
--isolated-script-test-chartjson-output flag (unused) so we don't
pass it on to the test executable.

[1]: https://chromium.googlesource.com/external/github.com/google/gtest-parallel/+/master/gtest-parallel
[2]: https://chromium.googlesource.com/external/webrtc/+/master/tools-webrtc/gtest-parallel-wrapper.py

BUG=709988
TBR=ehmaldonado@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2806373002
Cr-Commit-Position: refs/heads/master@{#17646}

tools-webrtc/gtest-parallel-wrapper.py

@@ -59,6 +59,11 @@ def main():
 
   parser = argparse.ArgumentParser()
   parser.add_argument('--isolated-script-test-output', type=str, default=None)
+
+  # TODO(ehmaldonado): Implement this flag instead of just "eating" it.
+  parser.add_argument('--isolated-script-test-chartjson-output', type=str,
+                      default=None)
+
   parser.add_argument('--output_dir', type=str, default=None)
   parser.add_argument('--timeout', type=int, default=None)