Notable changes:
* Allows macrobenchmarks to run via `flutter run`.
* Splits macrobenchmarking between "orchestration logic" and "app
serving" (served on separate ports on the same machine to keep the
scheme consistent with the `flutter build` path).
* Adds an intercepted entrypoint for web benchmarks. We can't pass flags
to the app since it's not supported, so I've hard-coded the
orchestration server's port.
* Adding logic to connect to an existing Chrome debugger instance (vs
spawning one) for benchmarks.
