SELECT element: Add HTMLSelectElement::selectedListIndex().

We had multiple instances of |optionToListIndex(selectedIndex())|, which causes
two iterations over OPTIONs. selectedListIndex() returns the same value, but is
more efficient.

Review-Url: https://codereview.chromium.org/2149763003
Cr-Commit-Position: refs/heads/master@{#405508}
4 files changed