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For each platform, we only use one overlay candidate type. Use build
flags to manage that. This also helps unify different platform's
ProcessForOverlay API.

R=rjkroege

Bug: 963597
Change-Id: Icf0de8b30928cb4e7c23a3573eb46591191fb804
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1664476
Commit-Queue: weiliangc <weiliangc@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680522}