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As of https://crrev.com/c/790511, this code isn't built on Android
anymore.
This also adds compile-time checks to many Instant headers to make sure
they're not accidentally built on Android.

Bug: 787830
Change-Id: I88026ce703158ac5b1580f02e621dea325a6fd64
Reviewed-on: https://chromium-review.googlesource.com/803280
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Chris Pickel <sfiera@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522101}