Port r10662 (c8054f6).

Original commit message:

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

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/9395015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10709 ce2b1a6d-e550-0410-aec6-3dcde31c8c00