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Forward drm and overlay command line flags for Guest session.

Bug: 
Change-Id: Ie9b167aefc7ab813116913534653903ccf5e323f
Reviewed-on: https://chromium-review.googlesource.com/572583Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487332}