Documentation: SkBitmap->SkImage

NOTRY=true
Change-Id: Ieb99a447b4198b1ba5d1bc17668b307d82d62be5
Reviewed-on: https://skia-review.googlesource.com/4121
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
2 files changed