This extends JSNativeContextSpecialization with support for stores to
fast object/smi element backing stores that are marked as copy-on-write.
In this case we first call the CopyFixedArray builtin to take a copy of
the elements backing store, and then store the new elements back to the
object, and finally perform the actual element store.

R=epertoso@chromium.org
BUG=v8:4470

Review-Url: https://codereview.chromium.org/2218703003
Cr-Commit-Position: refs/heads/master@{#38370}