Fixes https://github.com/flutter/flutter/issues/163479
This adds a flag,
`--cross-origin-isolation`/`--no-cross-origin-isolation` that allows the
user to explicitly control whether `flutter run`/`drive`/`test` serves
files with COOP/COEP headers. If the user doesn't specify, it uses cross
origin isolation when wasm is enabled and no cross origin isolation when
wasm is disabled.