Add 'WorkerGlobalScope::isSecureContext'

We shipped the 'Window' variant in [1] a ~year ago, with the promise
that we'd totally come back and do the 'Worker' bit. *cough* So, here
it is!

[1]: https://chromium.googlesource.com/chromium/src/+/fd67eed6357b2a5487c13cc159b4e843a9c91aee

BUG=534788,649896

Review-Url: https://codereview.chromium.org/2365353002
Cr-Commit-Position: refs/heads/master@{#421810}
12 files changed