Auto merge of #12404 - ConnorGBrewster:task_source_cleanup, r=asajeffrey

Clean up task sources and make all tasks cancellable

<!-- Please describe your changes on the following line: -->
This makes it so each task is a thin wrapper over a runnable and whenever a task is queued, it is automatically wrapped by the window's `runnable_wrapper`.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix  #11703 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/document.rs

@@ -1488,11 +1488,12 @@ impl Document {
 
         update_with_current_time_ms(&self.dom_content_loaded_event_start);
 
-        self.window().dom_manipulation_task_source().queue_event(self.upcast(), atom!("DOMContentLoaded"),
-            EventBubbles::Bubbles, EventCancelable::NotCancelable);
-        self.window().reflow(ReflowGoal::ForDisplay,
-                             ReflowQueryType::NoQuery,
-                             ReflowReason::DOMContentLoaded);
+        let window = self.window();
+        window.dom_manipulation_task_source().queue_event(self.upcast(), atom!("DOMContentLoaded"),
+            EventBubbles::Bubbles, EventCancelable::NotCancelable, window);
+        window.reflow(ReflowGoal::ForDisplay,
+                      ReflowQueryType::NoQuery,
+                      ReflowReason::DOMContentLoaded);
 
         update_with_current_time_ms(&self.dom_content_loaded_event_end);
     }