SharedWorker secure context integration.

[mikewest]

The SharedWorker() constructor algorithm is fairly different from the WHATWG version. In particular, I'm looking at the secure context integration in Step 11.4.2 of that algorithm which isn't present at all in this document. It landed in the WHATWG document a year and a half ago via whatwg/html#1560. It would be lovely if y'all would pull that update, along with the subsequent changes that resulted in the current text.

[ylafon]

See w3c/html#1075 and w3c/html#1090