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

This change switches ProfileSyncService tests to use ScopedTaskEnvironment
instead of MesageLoop because ModelTypeStore requires task scheduler that 
ScopedTaskEnvironment provides.

Also because DeviceInfoSyncBridge initializes ModelTypeStore before sync engine
initialization, test needs to pass path to valid temp dir to 
ProfileSyncService::InitParams::base_directory.

BUG=650725
R=skym@chromium.org

Change-Id: Id96d9028aa61bb275261e1452aa0dfec6381c651
Reviewed-on: https://chromium-review.googlesource.com/592134


Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491116}