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Previous CL added a SharedImageRepresentationTextureBase for other
reasons, use this to simplify SharedImageRepresentationSkiaGL.

Change-Id: Id0e027afc11c76b89664adc09b6ef3c85ac7a76c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1956255Reviewed-by: vikas soni <vikassoni@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728300}