Support non-random devtools port

[jdm]

A user reports that the randomized port makes the process of connecting from desktop Firefox more frustrating.

[jdm]

One possible solution:

• store the default debugger port as a preference
• if launching on that port fails, fall back to a random port
• update the preference when a new port is chosen

Another possible solution:

• store the debugger port in the UWP app settings
• when launching the embedding, if there's a port in the app settings pass the --devtools [port] argument

The second sounds easier.

[paulrouget]

if launching on that port fails, fall back to a random port

Do you remember why and how often that would happen?

[paulrouget]

I'm going for solution 1.

[jdm]

if launching on that port fails, fall back to a random port

Do you remember why and how often that would happen?

We added the random debugger port in response to a HoloLens OS update stealing port 6000 when we had that hardcoded, which caused FxR to panic during launch.