Add an about:memory page (#35728)

This patch exposes a servo internal DOM API that is only made available to about:
pages on the navigator object to request memory reports. The about:memory page itself is
loaded like other html resources (eg. bad cert, net error) and makes use of this new API.

On the implementation side, notable changes:
- components/script/routed_promise.rs abstracts the setup used to fulfill a promise when the
  work needs to be routed through the constellation. The goal is to migrate other similar
  promise APIs in followup (eg. dom/webgpu/gpu.rs, bluetooth.rs).
- a new message is added to request a report from the memory reporter, and the memory reporter
  creates a json representation of the set of memory reports.
- the post-processing of memory reports is done in Javascript in the about-memory.html page,
  providing the same results as the current Rust code that outputs to stdout. We can decide
  later if we want to remove the current output.

Signed-off-by: webbeef <me@webbeef.org>

components/script/dom/navigator.rs

@@ -27,6 +27,7 @@ use crate::dom::navigatorinfo;
 use crate::dom::permissions::Permissions;
 use crate::dom::pluginarray::PluginArray;
 use crate::dom::serviceworkercontainer::ServiceWorkerContainer;
+use crate::dom::servointernals::ServoInternals;
 #[cfg(feature = "webgpu")]
 use crate::dom::webgpu::gpu::GPU;
 use crate::dom::window::Window;
@@ -59,6 +60,7 @@ pub(crate) struct Navigator {
     gpu: MutNullableDom<GPU>,
     /// <https://www.w3.org/TR/gamepad/#dfn-hasgamepadgesture>
     has_gamepad_gesture: Cell<bool>,
+    servo_internals: MutNullableDom<ServoInternals>,
 }
 
 impl Navigator {
@@ -79,6 +81,7 @@ impl Navigator {
             #[cfg(feature = "webgpu")]
             gpu: Default::default(),
             has_gamepad_gesture: Cell::new(false),
+            servo_internals: Default::default(),
         }
     }
 
@@ -301,4 +304,10 @@ impl NavigatorMethods<crate::DomTypeHolder> for Navigator {
     fn HardwareConcurrency(&self) -> u64 {
         hardware_concurrency()
     }
+
+    /// <https://servo.org/internal-no-spec>
+    fn Servo(&self) -> DomRoot<ServoInternals> {
+        self.servo_internals
+            .or_init(|| ServoInternals::new(&self.global(), CanGc::note()))
+    }
 }