web: flamegraph: show filename in tooltip

Forked off from https://github.com/google/pprof/pull/649#issuecomment-930796262

### What version of pprof are you using?

7fe48b4c820be13151ae35ce5a5e3f54f1b53eef

### What did you do?

Open the flamegraph view in the web browser:


### What did you expect to see?
Filename in the tooltip -- in this case I used `-roottag thread` to add synthetic stack frames to the root of the trace, with the Filename = "thread", so help label the node.

### What did you see instead?
No filename.

Fixing this might require moving to a DOM-element for the tooltip, I'm not sure if browsers natively support multiline tooltips?