Use luci-go isolate client in mb.py

Bug: chromium:1059167
Change-Id: Idd3a3101ab09e9119bc99baab5a152dbd3ea6197
Reviewers: dpranke@chromium.org, tikuta@chromium.org
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170520
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Ye Kuang <yekuang@google.com>
Cr-Commit-Position: refs/heads/master@{#30796}

tools_webrtc/mb/mb_unittest.py

@@ -763,7 +763,7 @@ class UnitTest(unittest.TestCase):
     }
 
     def run_stub(cmd, **_kwargs):
-      if 'isolate.py' in cmd[1]:
+      if os.path.join('tools', 'luci-go', 'isolate') in cmd[0]:
         return 0, 'fake_hash base_unittests', ''
       else:
         return 0, '', ''