SELECT: Store the suggested value by an HTMLOptionElement pointer instead of an index number.

Managing an OPTION by an index number is a bad idea in general because an index
number is fragile.  This CL changes m_suggestedIndex from |int| to
|Member<HTMLOptionElement>|, and fix a bug about OPTION addition and removal.

Review-Url: https://codereview.chromium.org/2121083005
Cr-Commit-Position: refs/heads/master@{#403860}
5 files changed