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This allows ProfileSyncService to start up in "standalone transport"
mode even if the user has disabled Sync-the-feature. In particular,
the disable reasons USER_CHOICE and PLATFORM_OVERRIDE and the
FirstSetupComplete state don't prevent Sync-the-transport from
starting up.

Still missing:
- GetState should now be renamed to GetTransportState.
- UI code hasn't been updated, so UI might show wrong state.
- New integration tests.

Bug: 856179
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I59d3a4b548b279ec6cb5ca563747859dd9533618
Reviewed-on: https://chromium-review.googlesource.com/1148392Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580270}