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This is not used anywhere except in tests, so move it to the test code.
There's probably more to be done here; the test should probably mock GBM
instead of mocking dumb buffers, which have been completely removed
everywhere else.

Bug: 869206

Change-Id: I7900956d617d1341d222f0568516ae5027f6a9eb
Reviewed-on: https://chromium-review.googlesource.com/1155921
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579833}