SkImageInfo: make use of SkISize

Change-Id: Id3c427cae2988dcc2dfcf4dc713921142469e3f6
Reviewed-on: https://skia-review.googlesource.com/134840
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed