GitLab

* extracting CheckinRequest::RequestInfo to better manage checkin parameters
* adding G-services settings digest to RequestInfo of the checkin (will be sent with checkin request)
* extracting G-services settings from checkin response
* storing and loading of the G-services settings in GCM Store

This patch relands https://codereview.chromium.org/215363007/ after all
of the trybots, buildbots and CQ have been updated to run gcm_unit_tests

TBR=zea@chromium.org,jianli@chromium.org
BUG=359254

Review URL: https://codereview.chromium.org/233103003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263079 0039d316-1c4b-4281-b951-d872f2087c98