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_bindings/codegen/Bindings.conf

@@ -498,6 +498,11 @@ DOMInterfaces = {
     'canGc': ['Register'],
 },
 
+'ServoInternals': {
+    'inRealms': ['ReportMemory'],
+    'canGc': ['ReportMemory'],
+},
+
 'ShadowRoot': {
     'canGc': ['ElementFromPoint', 'ElementsFromPoint', 'SetInnerHTML'],
 },

components/script_bindings/webidls/ServoInternals.webidl

@@ -0,0 +1,19 @@
+/* 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/. */
+
+// Private interfaces that are only used for internal Servo usage
+// like about: pages.
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+[Exposed=Window,
+Func="dom::bindings::interface::is_servo_internal"]
+interface ServoInternals {
+    Promise<object> reportMemory();
+};
+
+partial interface Navigator {
+    [Func="dom::bindings::interface::is_servo_internal"]
+    readonly attribute ServoInternals servo;
+};