This CL uses UTF16 encoding internally in the string iterator,
thereby saving a few shifts, ors, and ands.

Bug: v8:7270
Change-Id: I3ac9e0e8c4b64eb1d6c796597eb0b3413c5f516b
Reviewed-on: https://chromium-review.googlesource.com/887085Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50904}