This adds an intermediate interface, nsIArrayExtensions, that inherits from
nsIArray. This is necessary as nsISupportsArray implements nsIArray as well
so the methods could not just be addded to nsIArray. nsIMutableArray inherits
from nsIArrayExtensions and so any interface that works with an nsIMutableArray
can be updated to return an nsIArrayExtensions.
This will allow interfaces that currently return an nsISupportsArray to instead
return an nsIArrayExtensions and internally work with an nsIMutableArray.
Consumers of these functions will continue to be able to use
nsISupportsArray-like iteration even though they're now working with an
nsIArray.
MozReview-Commit-ID: 9uRjsJbg9Jp
Eric Rahm