Index script compilation cache over context, too,

Index script compilation cache over context, too,
in preparation for global lexical scope.

R=ulan@chromium.org
BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10878007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00