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- Placed behind sync flag.
- Hooked up signinclient and tokenprovider to use CWVSyncController.
- Don't persist signin state between cold launches.

Bug: 867548
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Icf49e985aadec33b690552ff9973aadb4a5d1dba
Reviewed-on: https://chromium-review.googlesource.com/1211976
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594992}