Allow connection to parent process with launchpad

[bgrins]

If there is a bug that only gets hit in the Browser Toolbox, it's possible that the same bug could be hit and much easier to fix in the launchpad version of the tool if it could connect directly to the parent process. I'm thinking mostly of bugs blocking the dbg-browser and dbg-breakpoint meta.

Definitely seem easier than the hoops to jump through for actually debugging a browser toolbox i.e.:

./mach run --start-debugger-server 6081
MOZ_BROWSER_TOOLBOX_PORT=6081 ./mach run --profile /tmp/bt1 -chrome chrome://devtools/content/framework/toolbox-process-window.xul --start-debugger-server 6082
MOZ_BROWSER_TOOLBOX_PORT=6082 ./mach run --profile /tmp/bt2 -chrome chrome://devtools/content/framework/toolbox-process-window.xul