Merged FeedbackSlotInterface into AstNode, removing the need for a 2nd vtable.

This tiny change shaves off 112MB from the peak memory usage in the
bug mentioned below, more to come...

BUG=417697
LOG=y
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/611393004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24390 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Serialize all external strings except for native source code strings.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/604373008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24391 ce2b1a6d-e550-0410-aec6-3dcde31c8c00