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/shared/embedder/resources.rs

@@ -113,6 +113,8 @@ pub enum Resource {
     /// The page contains a js function `setData` that will then be used to build the list of directory.
     /// It can be empty but then nothing will be displayed when a directory listing is requested.
     DirectoryListingHTML,
+    /// A HTML page that is used for the about:memory url.
+    AboutMemoryHTML,
 }
 
 impl Resource {
@@ -132,6 +134,7 @@ impl Resource {
             Resource::MediaControlsJS => "media-controls.js",
             Resource::CrashHTML => "crash.html",
             Resource::DirectoryListingHTML => "directory-listing.html",
+            Resource::AboutMemoryHTML => "about-memory.html",
         }
     }
 }
@@ -189,6 +192,9 @@ fn resources_for_tests() -> Box<dyn ResourceReaderMethods + Sync + Send> {
                 Resource::DirectoryListingHTML => {
                     &include_bytes!("../../../resources/directory-listing.html")[..]
                 },
+                Resource::AboutMemoryHTML => {
+                    &include_bytes!("../../../resources/about-memory.html")[..]
+                },
             }
             .to_owned()
         }