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This CL adds a method to register a backing factory for testing
to SharedImageFactory, to be used in crrev.com/c/1619832. It is
needed since otherwise the GetFactoryByUsage() method starts
digging into the factory specifics and usually for testing this
is not needed and is cumbersome to mock.

Bug: 984836
Change-Id: I8009d54cb2a9576e10c5c63c803b9c6294b60f3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699531
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678338}