Debugger: ScopeMirror has N^2 algorithm when building closure mirrors.

R=yangguo@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/12326009
Patch from Pavel Feldman <pfeldman@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13699 ce2b1a6d-e550-0410-aec6-3dcde31c8c00