GitLab

[GCM] Adding GCMAccountMapper tests

1) When the status is ADDING (adding message is sent), the token is refreshed.
2) When the status is REMOVING (removing message is sent), the token is added
again.
3) updating multiple account mappings

BUG=374969
R=jianli@chromium.org

Review URL: https://codereview.chromium.org/544363002

Cr-Commit-Position: refs/heads/master@{#293744}