Regular map is preferred when parallization does
not speed presubmit tests.  It makes stacktraces more
understandable and facilitates debugging.  Only use
parallel pool when tests > 1.

R=maruel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15385002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@200940 0039d316-1c4b-4281-b951-d872f2087c98