[AF Profile Sync] Fix ios instrumentation tests for USS

This CL fixes an EG test on iOS that did not work well with the USS
implementation of sync. Concretely, it makes FakeServer provide the
right client tag hash when deleting an entity (which is required by
USS).

Bug: 836718
Change-Id: I4ca9ca1fedb2fd2d902d01a0a4cb9d67823ee2b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547988
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649432}