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For communication between the controller of a sync data type (running
always on the UI sequence) and the actual sync processor (running
potentially on backend sequences) we use proxy delegates that take care
of posting tasks across sequences. This is not needed for data types
that run on the UI sequence.

This CL makes use of the already existing
ForwardingModelTypeControllerDelegate for the USER_EVENTS sync data
type.

This has many caveats because the delegate gets fetched immediately
in some browser unit-tests and thus:
 - The FakeUserEventService has to provide a fake bridge and a fake
processor (that provides a null delegate). This in turn revealed a
previous BUILD.gn hack so the fake user event service needs to move
into a separate test_support build target.
 - on ios, the SessionSyncService has to provide a non-null
GlobalIdMapper to make the UserEventService constructor happy.

Bug: 867801
Change-Id: I47845732e9076e21544fba296323088750fff411
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1349690
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649812}