Replace WebContext::devTools* APIs with global API

[Chris Coulson]

Per-WebContext remote debugging hasn't worked correctly for a long time, since a Chromium change meant that all WebContents in a process were debuggable on all remote debugging HTTP endpoints.

Since https://chromium.googlesource.com/chromium/src.git/+/1062876ae55867edab8ec43884e9b9e15d0fe451%5E%21/, it is now no longer possible to set up multiple remote debugging HTTP endpoints. There can only be 1 per process, unless we make some changes to Chromium (and I don't think that's worth it).

For now, we'll have to modify Oxide to only allow remote debugging to be enabled on 1 WebContext at a time. We should deprecate this API though and replace it with a global one instead.