SELECT element: Introduce light-weight OPTION iterater.

blink::OptionList iterates over OPTION elements of a SELECT element without
producing vector-like cache. It's useful to avoid listeItems() in critical
paths.

This CL should not have user-visible behaivor changes.

BUG=577989

Review-Url: https://codereview.chromium.org/2130653002
Cr-Commit-Position: refs/heads/master@{#404100}
6 files changed