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enable_mac_keystone was a global flag in the build directory that's only used
by 4 total checks in 2 BUILD files. I'm trying to clean up the global build
flags.

Changes the checks to just check for mac Chrome branded builds. The previous
code additionally checked for "official" builds, but "official" is actually an
optimization level that I don't think is relevant to whether there are
installer scripts included in the bundle.

BUG=671706

Review-Url: https://codereview.chromium.org/2558973003
Cr-Commit-Position: refs/heads/master@{#437687}