script: Remove some proprietary Servo-only testing methods from Window (#42728)

These three methods date back from the very early history of Servo and
are no longer necessary:

1. `Window.debug`: `console.log` is a better replacement for this method
   now. A manual test that tests the very basics of JavaScript used
   this. This test is removed as well.
2. `Window.gc`: This can be replaced with `TestUtils.gc`, which is part
    of a W3C specification.
3. `Window.js_backtrace`: This method is moved to `ServoTestUtils`.

Testing: Tests are updated to reflect these changes.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/window.rs

@@ -15,7 +15,6 @@ use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 use app_units::Au;
-use backtrace::Backtrace;
 use base::cross_process_instant::CrossProcessInstant;
 use base::generic_channel::{self, GenericCallback, GenericSender};
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
@@ -942,6 +941,13 @@ impl Window {
             }),
         }
     }
+
+    #[expect(unsafe_code)]
+    pub(crate) fn gc(&self) {
+        unsafe {
+            JS_GC(*self.get_cx(), GCReason::API);
+        }
+    }
 }
 
 #[derive(Debug)]
@@ -1771,27 +1777,6 @@ impl WindowMethods<crate::DomTypeHolder> for Window {
     }
 
     // check-tidy: no specs after this line
-    fn Debug(&self, message: DOMString) {
-        debug!("{}", message);
-    }
-
-    #[expect(unsafe_code)]
-    fn Gc(&self) {
-        unsafe {
-            JS_GC(*self.get_cx(), GCReason::API);
-        }
-    }
-
-    #[expect(unsafe_code)]
-    fn Js_backtrace(&self) {
-        unsafe {
-            println!("Current JS stack:");
-            dump_js_stack(*self.get_cx());
-            let rust_stack = Backtrace::new();
-            println!("Current Rust stack:\n{:?}", rust_stack);
-        }
-    }
-
     fn WebdriverCallback(&self, realm: &mut CurrentRealm, value: HandleValue) {
         let webdriver_script_sender = self.webdriver_script_chan.borrow_mut().take();
         if let Some(webdriver_script_sender) = webdriver_script_sender {
@@ -3358,7 +3343,7 @@ impl Window {
         // GC any unreachable objects generated by user script,
         // or unattached DOM nodes. Attached DOM nodes can't be GCd yet,
         // as the document might be reactivated later.
-        self.Gc();
+        self.gc();
     }
 
     pub(crate) fn resume(&self, can_gc: CanGc) {