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>

ports/servoshell/egl/android/resources.rs

@@ -42,6 +42,9 @@ impl ResourceReaderMethods for ResourceReaderInstance {
             Resource::DirectoryListingHTML => {
                 &include_bytes!("../../../../resources/directory-listing.html")[..]
             },
+            Resource::AboutMemoryHTML => {
+                &include_bytes!("../../../../resources/about-memory.html")[..]
+            },
         })
     }