Add SK_API to SkROBuffer::Iter

Necessary for crrev.com/1812273003 in Chromium.

Also remove trailing whitespace.

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1856033002

Review URL: https://codereview.chromium.org/1856033002
1 file changed