[Worklets] Add basic debugging to main thread worklets.
This patch introduces MainThreadWorkletGlobalScope which lives in core/
This exposes accessing the parent LocalFrame for the MainThreadDebugger/devtools etc.
Devtools JS interprets this as a page script, will need to follow up with a sane UI for worklets in devtools :)
BUG=
567358
Committed:
https://crrev.com/9cdec5a9343fbba9ed72531696a3f4e06ebf4b6e
Cr-Commit-Position: refs/heads/master@{#382040}