There's a race condition in python's subprocess module, and gclient uses
it heavily while multithreaded. Avoid the race by locking around calls
to subprocess.Popen's constructor. Detailed explanation in the bug.

BUG=531561

Review URL: https://codereview.chromium.org/1343783004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296685 0039d316-1c4b-4281-b951-d872f2087c98