This templatizes CharacterStream by char type, and makes them subclass ScannerStream.
Methods that are widely used by tests are marked virtual on ScannerStream and final on
CharacterStream<T> so the specialized scanner will know what to call. ParseInfo passes
around ScannerStream, but the scanner requires the explicit CharacterStream<T>. Since
AdvanceUntil is templatized by FunctionType, I couldn't mark that virtual; so instead
I adjusted those tests to operate directly on ucs2 (not utf8 since we'll drop that in
the future).

In the end no functionality was changed. Some calls became virtual in tests. This is
mainly just preparation.

Change-Id: I0b4def65d3eb8fa5c806027c7e9123a590ebbdb5
Reviewed-on: https://chromium-review.googlesource.com/1156690
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54848}