[ChromeDriver] Build script report test failures

Currently, the ChromeDriver water shows green status even when some
tests have failed. Updating the build script to fix that.

Bug: 768880
Change-Id: I5f3ccadd00a119390dd9fbd625172f987a1100eb
Reviewed-on: https://chromium-review.googlesource.com/809726
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523552}

chrome/test/chromedriver/run_buildbot_steps.py

@@ -490,6 +490,8 @@ def main():
   # (which invoke this script) are kept in their own build step.
   util.MarkBuildStepStart('cleanup')
 
+  return 0 if passed else 1
+
 
 if __name__ == '__main__':
-  main()
+  sys.exit(main())