servo: Merge #15189 - Implement microtask checkpoints (from jdm:microtasks); r=nox

This generalizes the work previously done for Promise job callbacks. There is now a microtask queue that correctly processes all queued microtasks after each turn of the event loop, as well as after a scripted callback finishes executing, and after a classic script executes.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #4283
- [X] There are tests for these changes

Source-Repo: https://github.com/servo/servo
Source-Revision: cbcafd18f4cb5973948b081b4c104d99735e2789

servo/components/script/dom/htmlscriptelement.rs

@@ -486,11 +486,7 @@ impl HTMLScriptElement {
         document.set_current_script(Some(self));
 
         // Step 5.a.2.
-        let window = window_from_node(self);
-        let line_number = if script.external { 1 } else { self.line_number as u32 };
-        rooted!(in(window.get_cx()) let mut rval = UndefinedValue());
-        window.upcast::<GlobalScope>().evaluate_script_on_global_with_result(
-            &script.text, script.url.as_str(), rval.handle_mut(), line_number);
+        self.run_a_classic_script(&script);
 
         // Step 6.
         document.set_current_script(old_script.r());
@@ -506,6 +502,24 @@ impl HTMLScriptElement {
         }
     }
 
+    // https://html.spec.whatwg.org/multipage/#run-a-classic-script
+    pub fn run_a_classic_script(&self, script: &ClassicScript) {
+        // TODO use a settings object rather than this element's document/window
+        // Step 2
+        let document = document_from_node(self);
+        if !document.is_fully_active() || !document.is_scripting_enabled() {
+            return;
+        }
+
+        // Steps 4-10
+        let window = window_from_node(self);
+        let line_number = if script.external { 1 } else { self.line_number as u32 };
+        rooted!(in(window.get_cx()) let mut rval = UndefinedValue());
+        let global = window.upcast::<GlobalScope>();
+        global.evaluate_script_on_global_with_result(
+            &script.text, script.url.as_str(), rval.handle_mut(), line_number);
+    }
+
     pub fn queue_error_event(&self) {
         let window = window_from_node(self);
         window.dom_manipulation_task_source().queue_simple_event(self.upcast(), atom!("error"), &window);