[sync] Serve fake OAuth2 tokens from python sync server

This CL adds a new handler to the python sync server so it can serve
fake OAuth2 tokens to chrome. It is meant to be used for manual testing
with the python sync server, using the workflow described below:

- Build the "run_sync_testserver" target.
- Run the python sync server: "out/Debug/run_sync_testserver --port=8010"
- Launch chrome, sign in, and set up sync
- Re-launch chrome with "--lso-url=http://127.0.0.1:8010"

Chrome will now fetch OAuth2 tokens from the fake gaia server.

You may change the fake OAuth2 token returned by the python sync server
on the fly via the setfakeoauth2token handler like so:
http://127.0.0.1:8010/setfakeoauth2token?response_code=200&expires_in=2400&request_token=rt&access_token=at&token_type=Bearer

You may skip or include any of the query params. Params that are skipped
will remain unchanged.

BUG=none
TEST=Build and run the "run_sync_testserver" target. Test the /o/oauth2/token and /setfakeoauth2token handlers.
R=pavely@chromium.org

Review URL: https://codereview.chromium.org/58093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233271 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233271