This enables seamless unit tests that check-in files without side-effects on the
remaining unit tests.

BUG=none
TEST=none
TBR=dpranke

Review URL: http://codereview.chromium.org/6250073

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@73456 0039d316-1c4b-4281-b951-d872f2087c98