Exponential backoff and retry for MergeSession requests.
This CL makes the MergeSession requests within the GaiaCookieManagerService have automatic retry and exponential backoff functionality. This piece if prioritized ahead of most of the refactoring so it can get in for M43 branch point, since it will greatly smooth certain use cases on Android and iOS.
Current configuration will retry after: 1s, 3s, 9s, 27s, 1.4min (81s), 4min (243s), 12min (732s).
BUG=471210, 466799
Review URL: https://codereview.chromium.org/
1057973002
Cr-Commit-Position: refs/heads/master@{#323739}