Add getting the size of a given mipmap level.

When allocating memory for mipmaps, it would be very handy to know the
dimensions of a given mipmap level.

R=bsalomon@google.com
BUG=578304
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2018283002

Review-Url: https://codereview.chromium.org/2018283002
3 files changed