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This CL allows extensions to be marked as available on the 'lacros'
platform. This is primarily a small change to the generator python files
to accept a new enum type.

This CL marks enterprise.platformKeys as available on 'lacros' in order
to have real code using this logic path.

Change-Id: Icb593cb9fdd149154b7bd5d7206fad560ce82277
Bug: 1111879
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333570
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794821}