Take GrContext private, GrDDLContext inherit from GrRecordingContext

Woo!

Change-Id: I8d201b709343dc18cad31ea740575285dd035f35
Docs-Preview: https://skia.org/?cl=317436
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317436
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Adlai Holler <adlai@google.com>
11 files changed