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This copies the logic from tools/build/scripts/slave/runtest.py in swarming.

I changed test_env.py so that it sets CHROME_DEVEL_SANDBOX to be an empty string, instead of unsetting it. The latter doesn't work as Chrome triggers checks in content/browser/browser_main_loop.cc. This is what runtest.py does.

BUG=414808,336218

Review URL: https://codereview.chromium.org/605063004

Cr-Commit-Position: refs/heads/master@{#297330}