Do something about empty output directory fields in trace dialog

[ben-clayton]

This week I've had two reports of people unable to trace because they left the output directory field blank when attempting to take a trace.

For unix systems this attempts to write to the root directory: /foo.gfxtrace
For windows this attempts to write to the system32 directory: c:\windows\system32 (?!)

We should either prevent people from clicking okay, or default to a sensible location.