Lay some groundwork for depowering SkGpuDevice's and SkImage_GpuBase's context pointer

In future Ganesh these classes will, respectively, have a RecordingContext and an ImageContext.

Change-Id: I69ced004a2edae5c410e9c00c2de6c0246699caa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197167
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed