Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv

Rm readPixels from GrSurface & move read/writeSurfacePixels to GrContextPriv

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.

Change-Id: If57a7de47edc0853dae7bc61337d9acdc03d63b0
Reviewed-on: https://skia-review.googlesource.com/11200
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>