Refactor AO2TS to make it easier to componentize
This cl is dedicated to refactor token service to make it easier to componentize. It introduces a new class OAuth2TokenServiceDelegate for each platform to implement it. OAuth2TokenService and its subclasses will call into platform specific delegate and the user can get that delegate through public interface GetDelegate() of OAuth2TokenService.
BUG=490882
Review URL: https://codereview.chromium.org/
1143323005
Cr-Commit-Position: refs/heads/master@{#337248}