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Instead of partially initializing this in arcore_device, just pass the initial
object and let the GL thread take care of setting it up.

This also makes it possible to simplify mailbox_to_surface_bridge, it can now
assume that all post-creation methods are used on a single GL thread.

Add logic to enforce that at most one frame is allowed to be in "processing"
state, deferring the next frame in case of attempted overlap. This matches
the current logic used in GVR's immersive-vr mode.

Also update some stale comments and make simplifications that are now possible
due to the new lifecycle.

Bug: 1012972, 836524
Change-Id: I9b85bb6dd261c0c2fa41f443222e34b76a64c5c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854506
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705317}