The unit tests could modify global state in hard-to-revert ways and would make the PRESUBMIT.py check flaky.
Having the test running out of process alleviate any potential issue at the cost of speed (more noticeable on Windows).

TEST=unit tests
BUG=none

Review URL: http://codereview.chromium.org/147035

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@19248 0039d316-1c4b-4281-b951-d872f2087c98