Currently git-nav-downstream invokes 'git checkout' in a subprocess, but
exits without waiting for the checkout to complete. Instead wait for the
subprocess to complete.

BUG=376101

Review URL: https://codereview.chromium.org/509163002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291701 0039d316-1c4b-4281-b951-d872f2087c98