Cleanup scanner character streams.

Cleanup scanner character streams.

- GenericStringUtf16CharacterStream::start_position_ was unused.
- GenericStringUtf16CharacterStream inherits from BufferedUtf16CharacterStream,
so no need to initialize buffer_cursor_ and buffer_end_ twice (this makes it
clearer which class in the inheritance chain takes care of which variables).

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/216523004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00