MB: Add --quick flag for low_bandwidth_audio_test

This seems to be the preferred way to pass flags to tests running on Swarming.

Add --quick flag to the low_bandwidth_audio_test and support for 'args' keyword
(similar to upstream Chromium's mb.py).
Change the type of the test to non_parallel_console_test_launcher to avoid
running with gtest-parallel, since it will swallow the stdout output.

BUG=webrtc:7229
NOTRY=True
TBR=ehmaldonado@webrtc.org

Review-Url: https://codereview.webrtc.org/2763323003
Cr-Commit-Position: refs/heads/master@{#17353}
diff --git a/tools-webrtc/mb/mb.py b/tools-webrtc/mb/mb.py
index 1801a13..71fd7bf 100755
--- a/tools-webrtc/mb/mb.py
+++ b/tools-webrtc/mb/mb.py
@@ -1135,6 +1135,8 @@
           '--tsan=%d' % tsan,
       ]
 
+    cmdline += isolate_map[target].get('args', [])
+
     return cmdline, extra_files
 
   def ToAbsPath(self, build_path, *comps):