[iOS] Renaming SyncSwitchItem to LegacySyncSwitchItem

This CL prepares the GoogleSyncServicesViewController to be changed to
be used with UITableView.

Bug: 894791
Change-Id: I348cd9ee24695d7b07792b428035dfdf2f2a5a32
Reviewed-on: https://chromium-review.googlesource.com/c/1383034
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617949}
14 files changed