Switch SkImage's & SkImageGenerator's isValid call over to GrRecordingContext

This is another step towards replacing GrContext with the
GrDirectContext/GrRecordingContext pair.

Change-Id: I9336799a299e181fd56759f37dfed3da5da5d022
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301856
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
16 files changed