Simplify mach_override/BUILD.gn a bit.
Simplify mach_override/BUILD.gn a bit.

* assert(is_mac || is_ios) and assume is_clang is true, for
  consistency with ocmock/BUILD.gn
* Remove a duplicate -Wno-return-type flag

No intended behavior change.

BUG=none

Review URL: https://codereview.chromium.org/1559873002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367354}

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