Also make it possible to use the background parser from a character
stream. The External{One,Two}ByteStringUtf16CharacterStreams work both
on foreground and background threads.

BUG=v8:5215
R=marja@chromium.org,vogelheim@chromium.org

Review-Url: https://codereview.chromium.org/2195603002
Cr-Commit-Position: refs/heads/master@{#38162}