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This adds 2 entrypoints:
- CreateSharedImage, which takes a GpuMemoryBuffer to wrap into a shared
image (equivalent to CreateImageCHROMIUM+BindTexImage2DCHROMIUM)
- UpdateSharedImage, which updates the binding after CPU/external writes
to the GMB (equivalent to ReleaseTexImage2DCHROMIUM+BindTexImage2DCHROMIUM).

This is only plumbed to SharedImageBackingFactoryGLTexture in this CL.

Bug: 870116
Change-Id: Ie7645b65649b0f34d5431fc00f62da9d5661c570
Reviewed-on: https://chromium-review.googlesource.com/c/1316250Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605832}