Teach browsertime about startup performance tests

[dpalmeiro]

A possible setup here would be as follows:

1. Connect master and slave machines using an Avermedia capture card. Slave will be the reference device, master can be anything that is able to record from the capture card using ffmpeg.
2. Begin browsertime on master, and begin lightweight server on slave.
3. Browsertime on master begins recording, then sends remote command to slave to display orange frame (or other indicator of start time), and begins Firefox startup.
4. Master then ends recording after a set time interval, reboots slave (or flushes RAM) then repeats next iteration.
5. Master calculates visual metrics

This is a bit unusual because browsertime will not be running directly on the testing machine, but this is the ideal case for minimal overhead and faster turnaround since the slave will usually be a reference machine which is very slow.