Ensure that we don't mark weak heap references in the constant pool array.

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