Add addOnFlushCallbackObject at GrRecordingContext level The GrRecordingContext will (eventually) hold the drawManager so it is natural that it should accept onFlushCB objects Change-Id: I48dc876bd18e5d5ebea75c33498e75faba1a0feb Reviewed-on: https://skia-review.googlesource.com/c/193025 Auto-Submit: Robert Phillips <robertphillips@google.com> Commit-Queue: Greg Daniel <egdaniel@google.com> Reviewed-by: Greg Daniel <egdaniel@google.com>