[graphite] Make Context no longer ref counted.

Bug: skia:12794
Change-Id: I445144f25e9573d0d9ec55d12c3a9d372772c3bd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503348
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
11 files changed