Stop worrying about tiny OOMs. Work towards #20449.
While going through these, I also changed the function signature in many
places where returning ThrowCompletionOr<T> is no longer necessary.
This makes use of the new [UseNewAKString] extended attribute. Using
Vector storage will make it easier to make this interface into an IDL
iterable. It seems the reason it didn't use Vector originally was due
to awkward DeprecatedString -> String conversions.