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There were at least three different CallbackCounter classes
in the code base. Most of them counting the number of time
a callback has been invoked (and used only for tests) and
one invoking a OnceClosure after calls to DecrementCounter
matched earlier calls to IncrementCounter.

Since the class in base/ios/callback_counter.h had only a
single use, inline the code into the callsite and remove
the class that is confusing (the name does not really
correspond to the behaviour of the class and neither to
the other classes with the same name).

Other implementations are in:
- components/sync/test/callback_counter.h
- net/cookies/cookie_store_test_helpers.h

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I822243085afde29e640c72fd3e9790240a965987
Reviewed-on: https://chromium-review.googlesource.com/1020235
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552257}