PWA launch handler (navigate existing)

### Your use case

#### What would you like to do?

have the PWA manifest specify navigate-existing as launch handler client mode

#### Why would you like to do it?

because opening a new window for every link is stupid

#### How would you like to achieve it?

by changing manifest.json

### Have you considered any alternatives?

No

### Additional context

the launch handler feature ships publicly in next Chromium: https://chromestatus.com/feature/5722383233056768