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If a client embeds two different SurfaceIds with the same embed token,
we will end up in a bad state. Ignore duplicate embed tokens in
activation dependencies. I will investigate why the clients are
behaving this way but regardless of the reason viz should be resilient
and not crash.

Bug: 1001143,962367
Change-Id: I19dddbe60c2d87ce6b94a4fee4b584efafa70876
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791797
Commit-Queue: Saman Sami <samans@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694920}