Add GrTextureProxyPriv

Several upcoming additions should go in here

Change-Id: I642f3c7cc36b1e6512ee0170640449e88a666d2c
Reviewed-on: https://skia-review.googlesource.com/52661
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed