Make embedded maps in optimized code weak.

Each map has a weak array of dependent codes, where the map tracks all the optimized codes that embed it.
Old space GC either clears the dead dependent codes from the array if the corresponding map is alive or deoptimizes the live dependent codes if the map is dead.

BUG=v8:2073
R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/11575007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13490 ce2b1a6d-e550-0410-aec6-3dcde31c8c00