Add an into_repr on the nsstring bindings.

I didn't end up using this, but mystor reviewed it and it's useful.

MozReview-Commit-ID: 5LNzCEGpbLy

support/gecko/nsstring/src/lib.rs

@@ -559,6 +559,12 @@ macro_rules! define_string_types {
                     hdr: $StringRepr::new(ClassFlags::NULL_TERMINATED),
                 }
             }
+
+            /// Converts this String into a StringRepr, which will leak if the
+            /// repr is not passed to something that knows how to free it.
+            pub fn into_repr(mut self) -> $StringRepr {
+                mem::replace(&mut self.hdr, $StringRepr::new(ClassFlags::NULL_TERMINATED))
+            }
         }
 
         impl Drop for $String {