Add support for externally provided refresh tokens.

OAuth token cache file (as implemented by oauth2client library) stores refresh
token and can in theory be deployed via Puppet as the credential. But it is
mutated by the library (to store access tokens, rotated each hour), and so it is
not static and managing it via Puppet (or however else) is a big pain.

Instead, now depot_tools accepts --auth-refresh-token-json parameter with a path
to a static JSON file (with minimal body being {"refresh_token": "....."}). It
can be used to pass previously prepared refresh tokens of role accounts. It
will be used for blink DEPS roller account and similar @chromium.org accounts.

R=maruel@chromium.org
BUG=356813

Review URL: https://codereview.chromium.org/1060193005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294870 0039d316-1c4b-4281-b951-d872f2087c98

tests/git_cl_test.py

@@ -96,6 +96,7 @@ class TestGitCl(TestCase):
     self.mock(git_cl.upload, 'RealMain', self.fail)
     self.mock(git_cl.watchlists, 'Watchlists', WatchlistsMock)
     self.mock(git_cl.auth, 'get_authenticator_for_host', AuthenticatorMock)
+    self.mock(git_cl.auth, '_should_use_oauth2', lambda: False)
     # It's important to reset settings to not have inter-tests interference.
     git_cl.settings = None