Fuchsia provides a 'trace record' tool which we can use to create chrome:tracing compatible recordings of e.g. thread/process scheduling events, to help analyze slow-running tests.

The 'trace record' tool is now bundled into the Fuchsia SDK availabel to Chromium, so we should use it:
- Add an option to the 'runner' script(s) to run with tracing enabled.
- Enable the option on one or more flakey fixtures on the FYI bots, to gather data.