Remove unused arguments in HTMLSelectElement class.

- Remove |allowMultipleSelection| arguemnt of optionSelectedByUser().
  It's always false.

- Remove |shift| argument of listBoxSelectItem().
  It's always false.

Also, make optionSelectedByUser() private.

This CL has no behaivor changes.

Review-Url: https://codereview.chromium.org/2147683003
Cr-Commit-Position: refs/heads/master@{#405107}
3 files changed