Some PRESUBMIT files dynamically generate test list
and so may pass an empty list to RunTests.  There
appears to be a bug in multiprocessing.Pool that
causes a stall in map_async with an empty list.

TBR=maruel

Review URL: https://chromiumcodereview.appspot.com/14962011

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@200613 0039d316-1c4b-4281-b951-d872f2087c98