nacl: Be more consistent about setting nacl_untrusted_build on targets that use untrusted.gypi

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-Original-Commit-Position: refs/heads/master@{#348815}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cdde1a7e0f809dd3b368e5f8c48292bafdba9808