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Use the concrete type of the dumb buffer for cursors. We don't need
polymorphism here, and since cursors don't have framebuffers, this is
needed before evolving ScanoutBuffer into a concrete framebuffer type.

Bug: 869206

Change-Id: I5ee8162cd103d55df320cbc8e35be29e9ac15562
Reviewed-on: https://chromium-review.googlesource.com/1155911
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579535}