This fixes a corner case where weak function links of the code flushing
candidates list were destroyed by scavenges that happened during
incremental marking. Now those weak function links are updated while
scavenging happens.

R=ulan@chromium.org
TEST=cctest/test-heap/TestCodeFlushingIncrementalScavenge

Review URL: https://codereview.chromium.org/11271006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12825 ce2b1a6d-e550-0410-aec6-3dcde31c8c00