Rename GrTextureBufferSampler to GrBufferSampler

Just a simple renaming to reinforce the Ganesh notion that a buffer
access is not related to a texture.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4879

Change-Id: I3f12b6eccff744e246618be0a7abf41c4a1228a1
Reviewed-on: https://skia-review.googlesource.com/4879
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
6 files changed