Some heap pointer's embedded in optimized code are considered weak. Ensure
that we don't mark them during GC of the ConstantPoolArray.  Also, embed
length metadata in a bitfield, reducing the ConstantPoolArray header size from
five words to two.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/209473006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20573 ce2b1a6d-e550-0410-aec6-3dcde31c8c00