[graphite] Add async readpixels stubs to Image

Bug: b/238756873
Change-Id: I205b116bf38c853cbf66e53535d015b4ef18cb50
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/585966
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
2 files changed