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Because MockSyncSetupService is defined within the iOS libraries we can
define a static initializer that constructs a KeyedService using the
BrowserState. This will reduce the overhead for adding Sync services to
tests in iOS.

Bug: N/A
Change-Id: Ia142c70ebc605029c8677285f994151a1e1bc961
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575029Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835184}