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/script/script_msg.rs

@@ -27,6 +27,7 @@ use style_traits::CSSPixel;
 #[cfg(feature = "webgpu")]
 use webgpu::{WebGPU, WebGPUResponse, wgc};
 
+use crate::mem::MemoryReportResult;
 use crate::{
     AnimationState, AuxiliaryWebViewCreationRequest, BroadcastMsg, DocumentState,
     IFrameLoadInfoWithData, LoadData, MessagePortMsg, NavigationHistoryBehavior, PortMessageTask,
@@ -248,6 +249,8 @@ pub enum ScriptMsg {
     TitleChanged(PipelineId, String),
     /// Notify the constellation that the size of some `<iframe>`s has changed.
     IFrameSizes(Vec<IFrameSizeMsg>),
+    /// Request results from the memory reporter.
+    ReportMemory(IpcSender<MemoryReportResult>),
 }
 
 impl fmt::Debug for ScriptMsg {
@@ -308,6 +311,7 @@ impl fmt::Debug for ScriptMsg {
             GetWebGPUChan(..) => "GetWebGPUChan",
             TitleChanged(..) => "TitleChanged",
             IFrameSizes(..) => "IFramSizes",
+            ReportMemory(..) => "ReportMemory",
         };
         write!(formatter, "ScriptMsg::{}", variant)
     }