Import build/third_party when executing runtest.py.

Since runtest.py depends on modules listed in build/third_party,
we need to set --with-third-party-lib when we execute runtest.py
via runit.py.

Bug: 825290
Change-Id: Ic780388a9b9086364c1e4dcbd4a70a2cd2ed2425
Reviewed-on: https://chromium-review.googlesource.com/1053674Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557794}

testing/scripts/common.py

@@ -89,6 +89,7 @@ def run_runtest(cmd_args, runtest_args):
       sys.executable,
       cmd_args.paths['runit.py'],
       '--show-path',
+      '--with-third-party-lib',
       sys.executable,
       cmd_args.paths['runtest.py'],
     ]