Support using rr-record with headless servo

[jdm]

Running /.mach rr-record -z -d foo.html on a machine that can't create a window yields:

jdm@sandman:~/servo$ ./mach rr-record -z -d foo.html
rr: Saving execution to trace directory `/home/jdm/.local/share/rr/servo-0'.
assertion failed: !context.is_null() (thread main, at ports/servo/glutin_app/window.rs:98)
Error running mach:

    ['rr-record', '-z', '-d', 'foo.html']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.

You should consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

CalledProcessError: Command 'rr --fatal-errors record /home/jdm/servo/target/debug/servo -z foo.html' returned non-zero exit status 101

  File "/home/jdm/servo/python/servo/post_build_commands.py", line 210, in rr_record
    check_call(rr_cmd + servo_cmd)
  File "/home/jdm/servo/python/servo/command_base.py", line 179, in check_call
    raise subprocess.CalledProcessError(status, ' '.join(*args))

[highfive]

cc @wafflespeanut

[mateon1]

Using ./mach run [-r|-d] -z --debugger=rr ... works. Looks like rr-record doesn't set the proper environment variables for headless mode.