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/servointernals.rs

@@ -0,0 +1,62 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use std::rc::Rc;
+
+use dom_struct::dom_struct;
+use profile_traits::mem::MemoryReportResult;
+use script_traits::ScriptMsg;
+
+use crate::dom::bindings::codegen::Bindings::ServoInternalsBinding::ServoInternalsMethods;
+use crate::dom::bindings::error::Error;
+use crate::dom::bindings::reflector::{DomGlobal, Reflector, reflect_dom_object};
+use crate::dom::bindings::root::DomRoot;
+use crate::dom::globalscope::GlobalScope;
+use crate::dom::promise::Promise;
+use crate::realms::InRealm;
+use crate::routed_promise::{RoutedPromiseListener, route_promise};
+use crate::script_runtime::CanGc;
+
+#[dom_struct]
+pub(crate) struct ServoInternals {
+    reflector_: Reflector,
+}
+
+impl ServoInternals {
+    pub fn new_inherited() -> ServoInternals {
+        ServoInternals {
+            reflector_: Reflector::new(),
+        }
+    }
+
+    pub(crate) fn new(global: &GlobalScope, can_gc: CanGc) -> DomRoot<ServoInternals> {
+        reflect_dom_object(Box::new(ServoInternals::new_inherited()), global, can_gc)
+    }
+}
+
+impl ServoInternalsMethods<crate::DomTypeHolder> for ServoInternals {
+    /// <https://servo.org/internal-no-spec>
+    fn ReportMemory(&self, comp: InRealm, can_gc: CanGc) -> Rc<Promise> {
+        let global = &self.global();
+        let promise = Promise::new_in_current_realm(comp, can_gc);
+        let sender = route_promise(&promise, self);
+        let script_to_constellation_chan = global.script_to_constellation_chan();
+        if script_to_constellation_chan
+            .send(ScriptMsg::ReportMemory(sender))
+            .is_err()
+        {
+            promise.reject_error(Error::Operation, can_gc);
+        }
+        promise
+    }
+}
+
+impl RoutedPromiseListener for ServoInternals {
+    type Response = MemoryReportResult;
+
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+    fn handle_response(&self, response: Self::Response, promise: &Rc<Promise>, can_gc: CanGc) {
+        promise.resolve_native(&response.content, can_gc);
+    }
+}