GitLab

Adding registration ID request and tests to GCM Account Mapper
* Adding code to get registration ID from GCM
* Making sure account tokens are only processed when reg ID is present
* Remembering only the last assigned set of account tokens
* Fixing tests that require registration ID
* Adding tests specific to registration ID setting

BUG=374969
R=zea@chromium.org

Review URL: https://codereview.chromium.org/580603002

Cr-Commit-Position: refs/heads/master@{#295890}