Fix huge heap snapshot when a heavily shared context has many variables

Prevously v8 put a link to each context variable into a function where
the variable is visible.
Because of that if there are N functions sharing a context having M variables
then N*M links were created for the snapshot.
The fix makes v8 to put the links into the context object.

BUG=145687
TEST=test-heap-snapshot/ManyLocalsInSharedContext

Review URL: https://codereview.appspot.com/7715044

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13936 ce2b1a6d-e550-0410-aec6-3dcde31c8c00