Do not compact weak fixed array when re-allocating new backing store.

Do not compact weak fixed array when re-allocating new backing store.

JSObject::UnregisterPrototypeUser relies on the index of the weak
fixed array entry not to change.

See commit 1e65e2018.

There is no bug currently, because we would only re-allocate if there is no empty slot left. In which case there is no actual compaction going on during re-allocation.

R=jkummerow@chromium.org, mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1294883004

Cr-Commit-Position: refs/heads/master@{#30251}