Make sure that mb can pull out the args from script test target.

Context: this is change is split from
https://codereview.chromium.org/1354223004/, which introduce
telemetry_gpu_unittests, which a script target that has args.

BUG=507796

Review URL: https://codereview.chromium.org/1376823002

Cr-Commit-Position: refs/heads/master@{#351373}
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index 12f942d..c6e66e74 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -557,7 +557,8 @@
       ]
       cmdline = [
           '../../testing/test_env.py',
-      ] + ['../../' + self.ToSrcRelPath(gn_isolate_map[target]['script'])]
+          '../../' + self.ToSrcRelPath(gn_isolate_map[target]['script'])
+      ] + gn_isolate_map[target].get('args', [])
     elif test_type in ('raw'):
       extra_files = []
       cmdline = [