Support browser's profile viewers json formats

[dbuenzli]

Speaking of viewers, web browsers come with their own profile viewers. Maybe you could simply piggyback on these.

For Chrome this happens at the chrome://tracing address and the JSON file format it can read is described here.

For Firefox it's in the browser's webdeveloper performance tools or here. The format seems to be a little bit less well documented but some type defintions can be found here.