[Cocoa] Wrappers' -copyWithZone: should copy if the collection is mutable

[Cocoa] Wrappers' -copyWithZone: should copy if the collection is mutable
https://bugs.webkit.org/show_bug.cgi?id=123707

Reviewed by Geoff Garen and Anders Carlsson.

* Shared/Cocoa/WKNSArray.mm:
(-[WKNSArray copyWithZone:]): If the array is mutable, make a copy.
* Shared/Cocoa/WKNSDictionary.mm:
(-[WKNSDictionary copyWithZone:]): If the dictionary is mutable, make a copy.
* Shared/ImmutableArray.h:
(WebKit::ImmutableArray::entries): Added this accessor.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158553 268f45cc-cd09-0410-ab3c-d52691b4dbfc