A sizes attribute parser

This CL implements a parser for HTMLImageElement's sizes attribute.
It also hooks that parser to HTMLImageElement, but doesn't yet do anything with the output.

BUG=357586

Review URL: https://codereview.chromium.org/224733011

git-svn-id: svn://svn.chromium.org/blink/trunk@171701 bbb929c8-8fbe-4397-9dbb-9b2b20218538