Skip write barriers when updating the weak hash table.

Write barrier on the weak hash table makes all its pointers strong,
which can cause a memory leak.

BUG=359401
LOG=Y
TEST=cctest/test-heap/NoWeakHashTableLeakWithIncrementalMarking
R=hpayer@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/284773004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21299 ce2b1a6d-e550-0410-aec6-3dcde31c8c00