Visit stored context when iterating stack for optimized stub.

BUG=v8:2509
TEST=mjsunit/regress/regress-1125,NavierStokes fails on Nexus 10.
R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/12314124

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13733 ce2b1a6d-e550-0410-aec6-3dcde31c8c00