SkMipMap::ComputeLevelSize to return SkISize

This previously returned SkSize, which had scalar components.
That doesn't make sense for a mipmap size.

R=bsalomon@google.com
BUG=578304
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2036313002

Review-Url: https://codereview.chromium.org/2036313002