legacy platform objects no longer override desc.[[Configurable]]

None of the code generation for named setters in any of the major browsers appears to honor Step 3. of 3.9.3. [DefineOwnProperty].

The test case in WPT (http://wpt.live/dom/collections/HTMLCollection-supported-property-names.html) already appears to assert that the descriptor is not forced to become [[Configurable]], and this test case is passing in most implementations.

I've done a bunch of detective work to justify this 2 line patch, so dumping all of that here. commit hashes are current as of writing this.

With the [[GetOwnProperty]] algorithm, there are a few documented instances where this is intentionally ignored:

[[GetOwnProperty]] for Location objects: https://github.com/mozilla/gecko-dev/blob/0b1d02b2cb5736511139cf0e40b318273e825899/dom/bindings/Codegen.py#L16396-L16400 -- Per Gecko's passing the test, it does not appear to actually override the set descriptor in any way, at least not with regards to configurability.

Finally, in WebKit we also see no overriding behaviour for the descriptor attributes when the named property visibility algorithm returns false: https://github.com/WebKit/WebKit/blob/f288b9088d1b74b980c3b7a8d5add134f7689bdb/Source/WebCore/bindings/scripts/CodeGeneratorJS.pm#L983-L992

[x] At least two implementers are interested (and none opposed):
- N/A, already implemented
[x] Tests are written and can be reviewed and commented upon at:
- N/A, already tested
[x] Implementation bugs are filed:
- N/A, already implemented
[x] MDN issue is filed: N/A
[x] The top of this comment includes a clear commit message to use.

whatwg / webidl