Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv (take 3)

Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv (take 3)

This is in service of: https://skia-review.googlesource.com/c/11125/ (Add parallel proxyID to StencilOps & RenderTargetOpList) where I want a better choke point for texture creation to improve discard handling.

This is a re-reland of: https://skia-review.googlesource.com/c/11200/ (Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv)

Change-Id: Icfb9dd223418dd460405efd2bfd9d1c356beed1a
Reviewed-on: https://skia-review.googlesource.com/11412
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>