Too many files know how to deal with decoding, counting, and splitting UTF-8
into uc16 chars. This consolidates several callers who deal with full
(Vector<char>, not streaming) bytes by using a UTF-8 Iterator to decode bytes
into individual uc16 chars.

R=marja@chromium.org

Bug: 
Change-Id: Ia36df3e8c1abd0398415ad23a474557c71c19a01
Reviewed-on: https://chromium-review.googlesource.com/831093Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Justin Ridgewell <jridgewell@google.com>
Cr-Commit-Position: refs/heads/master@{#51405}