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/constellation/constellation.rs

@@ -1735,6 +1735,11 @@ where
                 }
             },
             FromScriptMsg::IFrameSizes(iframe_sizes) => self.handle_iframe_size_msg(iframe_sizes),
+            FromScriptMsg::ReportMemory(sender) => {
+                // get memory report and send it back.
+                self.mem_profiler_chan
+                    .send(mem::ProfilerMsg::Report(sender));
+            },
         }
     }