Arenas struct for GrRecordingContext

Change-Id: If05370b0970af5daf976d60e1fc97f2fab3c213e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261082
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed