isTrusted was hardcoded to true for every Event, so new Event('x').isTrusted
returned true. Real browsers return false for script-constructed events and
true only for user-agent dispatched ones, so this was a trivial bot-detection
tell (issue #303).
Track trusted events in a closure-private WeakSet that page JS cannot read or
forge; default isTrusted to false. The CDP input pipeline marks its synthesized
mouse/key events trusted via a non-enumerable __obscura_markTrusted helper, so
automation clicks stay trusted like real Chrome over CDP while page-built
events report false.