Prevent excessive processing of weak maps while marking.

This makes sure processing of Harmony weak maps is only triggerd twice
during a mark-compact collection. Note that the processing itself still
is a fix-point iteration with quadratic worst-case time.

R=hpayer@chromium.org
BUG=v8:2699

Review URL: https://codereview.chromium.org/16286018

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14940 ce2b1a6d-e550-0410-aec6-3dcde31c8c00