Turns Declaration into an abstract class, and introduces VariableDeclaration as a concrete subclass.

R=kmillikin@chromium.org
BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/9348057

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10662 ce2b1a6d-e550-0410-aec6-3dcde31c8c00