Add --retry_failed=3 flag to gtest-parallel.

This means the test will be executed 4 times max, one run and three retries.

BUG=chromium:718339

Review-Url: https://codereview.webrtc.org/2861043002
Cr-Commit-Position: refs/heads/master@{#18028}
diff --git a/tools-webrtc/mb/mb.py b/tools-webrtc/mb/mb.py
index a2a593f..bbbf413 100755
--- a/tools-webrtc/mb/mb.py
+++ b/tools-webrtc/mb/mb.py
@@ -1102,6 +1102,7 @@
             # so it can exit cleanly and report results, instead of being
             # interrupted by swarming and not reporting anything.
             '--timeout=900',
+            '--retry_failed=3',
         ]
 
       asan = 'is_asan=true' in vals['gn_args']