More context in the #44878. But generally. I wanted to be able to run

navigator.servo.garbageCollectAllContexts()

In the blink-perf-test's runner.js so that it does not use the strange fallback method.

function gcRec(n) {
     if (n < 1)
         return {};
     var temp = {i: "ab" + i + (i / 100000)};
     temp += "foo";
     gcRec(n-1);
 }
 for (var i = 0; i < 1000; i++)
     gcRec(10);

So now, this PR introduces --pref dom_servo_gc_enabled=true
That makes is_servo_internal in the components/script/dom/servointernals.rs to return true, so that the navigator.servo.garbageCollectAllContexts() could be called outside the about:memory url.

Testing: To test, I've locally put a print! in the fn GarbageCollectAllContexts(&self) that is in the components/script/dom/servointernals.rs and run the servo with and without the --pref dom_servo_gc_enabled=true. As expected, the print! worked only when the correct --pref is present.

Fixes: #44878