Make maps in monomorphic IC stubs weak.

Maps in monomorphic Load, KeyedLoad, Store, KeyedStore, and CompareNil IC
stubs are treated as weak references by the marking visitor.

During generation of an IC stub with a weak map, the stub is appended to the
dependent code array of the map. When the map dies, all stubs in its dependent
code array are invalidated by setting embedded maps to undefined.

BUG=v8:2073
LOG=Y
TEST=cctest/test-heap/WeakMapInMonomorphic*IC
R=mstarzinger@chromium.org, verwaest@chromium.org

Review URL: https://codereview.chromium.org/188783003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20679 ce2b1a6d-e550-0410-aec6-3dcde31c8c00