Parser / Scanner: Minor refactorings to make streaming scripts work easier.

1) Call DeserializeScopeChain only if it's going to do something
non-trivial. And we only need to internalize the AstValueFactory in those cases.

2) BufferedUtf16CharacterStream::FillBuffer doesn't need the length
argument. The length is always kBufferSize and the subclasses can just read it
(it's protected).

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/381613003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22307 ce2b1a6d-e550-0410-aec6-3dcde31c8c00