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Updates Mac and Linux installers to place manifests in proper per-
platform location. Windows is more complicated due to 32/64-bit registry
considerations, so the installer will be updated in a follow-up commit.

Change-Id: If2f6a4f77e013703c146bba91aaa4362eea12e11
Reviewed-on: https://chromium-review.googlesource.com/1187659
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586017}