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The motivation is that I want to add a certain include_dir to all targets (to
use libc++ on OS X), _except_ for NaCl targets, since these have their own
bundled C++ standard library already.  Checking for nacl_untrusted_build almost
works, but a handful nacl targets didn't set that yet.  Fix this.  No intended
behavior change.

BUG=400091

Review URL: https://codereview.chromium.org/1332163005

Cr-Commit-Position: refs/heads/master@{#348815}