[Sync] Enable USS implementation of Autocomplete by default

USS Autocomplete is being launched on stable. It needs to be enabled by
default on ToT.

BUG=702711
R=pnoland@chromium.org

Change-Id: I10b32e8d948728ef8f4d98841639e6e862378a4d
Reviewed-on: https://chromium-review.googlesource.com/779633


Reviewed-by: Patrick Noland <pnoland@google.com>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518268}