unify how canvas checks for null on skia objects (images, blobs, pictures)

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1568613008

Review URL: https://codereview.chromium.org/1568613008
2 files changed