Auto merge of #23080 - jdm:sampling-profiler, r=gterzian

Add a sampling profiler

This uses the code already built for the background hang monitor and adds the ability to repeatedly sample all monitored threads. This sampling allows us to generate profiles that we can translate into the format used by https://perf-html.io/, allowing us to benefit from modern Gecko performance tooling.

You can run Servo with `PROFILE_OUTPUT=foo.json` and `SAMPLING_RATE=50` (for example), otherwise these values will default to `samples.json` and 10ms, respectively. To activate the profiler, press cmd+p, and to stop profiling, press cmd+p again. This will the captured samples to be symbolicated, which will take a very long time, and eventually there will be a new JSON profile in the output location.

To create a profile for use by Gecko's tools, run `python etc/profilicate.py path/to/profile.json >gecko_profile.json`, and load `gecko_profile.json` in the https://perf-html.io/ to see something like [this](https://profiler.firefox.com/public/8137e2b11fbb92afb80090bc534fd83015c87ee6/calltree/?globalTrackOrder=0-1&thread=1&v=3);

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13103
- [x] These changes do not require tests because way too many pieces to automate

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23080)
<!-- Reviewable:end -->

components/compositing/windowing.rs

@@ -16,6 +16,7 @@ use servo_url::ServoUrl;
 use std::fmt::{Debug, Error, Formatter};
 #[cfg(feature = "gl")]
 use std::rc::Rc;
+use std::time::Duration;
 use style_traits::DevicePixel;
 use webrender_api::{
     DeviceIntPoint, DeviceIntRect, DeviceIntSize, DevicePoint, FramebufferIntRect,
@@ -94,6 +95,8 @@ pub enum WindowEvent {
     ToggleWebRenderDebug(WebRenderDebugOption),
     /// Capture current WebRender
     CaptureWebRender,
+    /// Toggle sampling profiler with the given sampling rate
+    ToggleSamplingProfiler(Duration),
 }
 
 impl Debug for WindowEvent {
@@ -121,6 +124,7 @@ impl Debug for WindowEvent {
             WindowEvent::SelectBrowser(..) => write!(f, "SelectBrowser"),
             WindowEvent::ToggleWebRenderDebug(..) => write!(f, "ToggleWebRenderDebug"),
             WindowEvent::CaptureWebRender => write!(f, "CaptureWebRender"),
+            WindowEvent::ToggleSamplingProfiler(..) => write!(f, "ToggleSamplingProfiler"),
         }
     }
 }