Enable parallel tests on depot_tools.
On Z620 presubmit times: 3m -> 35s.

Review URL: https://codereview.chromium.org/14247012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@195377 0039d316-1c4b-4281-b951-d872f2087c98