Improve performance of HTMLSelectElement::deselectItemsWithoutValidation().

This CL improves it in a case of single-selection menulist SELECTs.

blink_perf.dom select-single-add.html:
  585.9 runs/s -> 655.3 runs/s

BUG=577989

Review-Url: https://codereview.chromium.org/2123863002
Cr-Commit-Position: refs/heads/master@{#403856}
1 file changed