GitLab

- Add a fetcher class, OAuth2TokenMintFetcher, to abstract OAuth2 token mint process.
- Add tests.

The work to use this in AppNotifyChannlesetup will be done in upcoming patch.
Review URL: https://chromiumcodereview.appspot.com/9549013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@124486 0039d316-1c4b-4281-b951-d872f2087c98