Runtime.executionContextsCleared on navigation told clients every prior
context was gone, but valid_context_ids was insert-only, so a
Runtime.callFunctionOn targeting a pre-navigation context id still ran
instead of being rejected with Chrome's 'Cannot find context with
specified id'. On a long-lived CDP connection the set also grew by one
id per navigation and was never pruned.
Clear valid_context_ids when executionContextsCleared is emitted, then
re-register the default world (id 2) and the isolated worlds as their
executionContextCreated events are emitted. Closes#407.
