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/tracing.rs

@@ -185,6 +185,7 @@ mod from_script {
                 Self::GetWebGPUChan(..) => target!("GetWebGPUChan"),
                 Self::TitleChanged(..) => target!("TitleChanged"),
                 Self::IFrameSizes(..) => target!("IFrameSizes"),
+                Self::ReportMemory(..) => target!("ReportMemory"),
             }
         }
     }