This avoids the need to pull in the UTF-8 encoding code from the public API,
and allows it to take advantage of any supported way that i::String can be
encoded (one- or two-byte).

Backward compatibility is maintained, but this is the behavior beginning
with this version.

BUG=chromium:686159

Review-Url: https://codereview.chromium.org/2665653004
Cr-Commit-Position: refs/heads/master@{#42872}