JavaScriptScanner had become the only concrete subclass of Scanner, so there
was no longer a need for the distinction.

Also fixed up comments.

Review URL: http://codereview.chromium.org/8384003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9854 ce2b1a6d-e550-0410-aec6-3dcde31c8c00