Convert GrBufferType to enum class, rename, and remove dead values

Renamed to GrGpuBufferType in anticipation of splitting GrBuffer
into GrGpuBuffer and GrCpuBuffer types.

There were two unused values in the enum that are removed, DrawIndirect
and Texel.

Change-Id: Icb6b3da689adbd8e10495c10fd0470a6ee0120b5
Reviewed-on: https://skia-review.googlesource.com/c/189280
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
45 files changed