bug 1092231 - fix traits to work with Symbols

after some discussion in #jsapi and reading the [es6 spec (draft)](https://people.mozilla.org/~jorendorff/es6-draft.html), i'm gonna disagree with this.

a `"Key"` is a commonly used term thru-out the es6 spec to signify both String and Symbol-valued property keys. there is even an internal method with (almost) the same name i proposed [[[OwnPropertyKeys]]](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-ordinary-object-internal-methods-and-internal-slots-ownpropertykeys), and various public-api references to "Keys" as representing both String and Symbol values (in [Proxy handler trap](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-proxy-object-internal-methods-and-internal-slots), [Reflect.ownKeys](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-reflect.ownkeys), etc). the only exception is indeed Object.keys (compat reasons obviously), but that's an inconsistency on the language-level, not one we are introducing with this API. IMHO, using "MemberNames" to represent both String and Symbol values is even more misleading, and goes directly against the spec (term "Names" is never used for non-String values).

and second, regardless of the name, a generalized method like you are suggesting doesn't seem too widely useful imho:
 - it's mixing and sometimes returns only own, and other times all (enumerable, string) property keys. most of the time, at a single point in code, you are either interested in all or only own properties.
 - mixing { keys: true, symbols: true } would return all String-keyed properties, but only own Symbol-keyed ones (i doubt that would ever be useful).
 - it's too verbose way to do any other task (except the one i specifically need in this case). why would you a) import the method from the module, and b) call `getMemberNames(obj, { params: true })` in any other case (except this specific one), when every other use case is covered and available as an existing single method call on the Object global.

so, if you still disagree with my method name/definition as a public exposed api, i'm fine with just defining and using it as a local helper function in the two modules that need it.