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As iOS has fully migrated to ARC (automatic reference counting)
and there is no traction to migration macOS to ARC, remove the
support allowing to use scoped_{nsobject,block} in files build
with ARC.

The guards are not necessary as scoped_{nsobject,block} won't
build with ARC since they contains calls to -release and cast
blocks to void*. However, they make it easier to understand
that this is not supported, so add them for documentation.

Bug: 1051997
Change-Id: I3db04ec645cb1830b92003723631c119e6e4a94c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2082385
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746425}