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When targeting iOS, the host platform is macOS but only tools are
built, so unconditionally disable nacl for both the target and the
host (i.e. check target_os instead of is_ios).

Also move the check for compatibility between the version of the
OS used for the build and NaCL from build/config/mac/mac_sdk.gni
to components/nacl/features.gni (the error is specific to NaCL,
not to any project that want to reuse Chrome build system).

Bug: 1049832
Change-Id: Id8efb5b4b271a7a6164333efc09f24804564c7a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088102
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747235}