Added automatic detection of OAuth2 token expiracy. Token is
automatically renewed when necessary.

BUG=363825

Review URL: https://codereview.chromium.org/243323003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@264871 0039d316-1c4b-4281-b951-d872f2087c98