[DevTools] Move WorkerInspectorController to WorkerThread.
This allows to unify notifications flushing with main thread inspector.
It also makes possible to support multiple workers/worklets sharing the same
isolate in the future.
BUG=none
Committed:
https://crrev.com/369e46c8b42e8caf633f02f034d6ef95d1424a41
Cr-Commit-Position: refs/heads/master@{#409441}