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Change DrmFramebuffer from a virtual interface to a concrete wrapper for
a DRM framebuffer. This is what used to be ScanoutBuffer and is all that
is needed for modesetting. In particular, we don't need access to gbm_bo,
skia surfaces backed by dumb buffer mmaps, or other bits from
{Gbm,Drm}Buffer in the modesetting internals.

DrmFramebuffer is composed into GbmBuffer and DrmBuffer so that we can
still access framebuffers for those types when presenting a buffer.
GbmBuffer and DrmBuffer are no longer refcounted after this change.

Bug: 868010

Change-Id: I6d31b000b35bc4a6caa6280bee4156ca9b271bb9
Reviewed-on: https://chromium-review.googlesource.com/1155916
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579605}