Bug 1431204 - Change calls to nsIURI.spec setter to use nsIURIMutator instead r=mayhemer

* changes call to use nsIURIMutator.setSpec()
* Add new NS_MutateURI constructor that takes new Mutator object
* Make nsSimpleNestedURI::Mutate() and nsNestedAboutURI::Mutate() return mutable URIs
* Make the finalizers for nsSimpleNestedURI and nsNestedAboutURI make the returned URIs immutable

MozReview-Commit-ID: 1kcv6zMxnv7

netwerk/base/nsSimpleNestedURI.cpp

@@ -196,6 +196,9 @@ nsSimpleNestedURI::Mutate(nsIURIMutator** aMutator)
     if (NS_FAILED(rv)) {
         return rv;
     }
+    // StartClone calls SetMutable(false) but we need the mutator clone
+    // to be mutable
+    mutator->ResetMutable();
     mutator.forget(aMutator);
     return NS_OK;
 }