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=

Committed: https://code.google.com/p/v8/source/detail?r=20334

[marja, 2014-03-28 13:00:09]

yangguo, again a tiny cleanup CL, ptal.

[Yang, 2014-03-28 13:00:38]

lgtm

[marja, 2014-03-28 13:11:18]

Committed patchset #1 manually as r20334 (presubmit successful).