- The static method CopyChars was actually used and has been extracted.
- It was used in tests, where it's been replaced w/ ExternalOneByteString...
- Only one test actually relied on Utf8 handling (as opposed to ASCII only),
  and that was the test testing Utf8ToUtf16CharacterStream itself.

+66 -277 LOC :)

BUG=v8:4947

Review-Url: https://codereview.chromium.org/2256273002
Cr-Commit-Position: refs/heads/master@{#38824}