Auto merge of #16963 - BorisChiou:stylo/animation/omta, r=emilio

stylo: Bug 1334036 - Enable animations running on compositor.

These are interdependent patches of Bug 1334036, which enables off-main thread animations. We add one FFI to get the property id set which overriding animations, so we can make sure the cascade result is correct for off-main thread animations.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1334036](https://bugzilla.mozilla.org/show_bug.cgi?id=1334036)
- [X] These changes do not require tests because we support off-main thread animation only on Gecko, and there are enough test cases there.

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components/script/layout_wrapper.rs

@@ -486,7 +486,10 @@ impl<'le> TElement for ServoLayoutElement<'le> {
     }
 
     fn has_animations(&self) -> bool {
-        unreachable!("this should be only called on gecko");
+        // We use this function not only for Gecko but also for Servo to know if this element has
+        // animations, so we maybe try to get the important rules of this element. This is used for
+        // off-main thread animations, but we don't support it on Servo, so return false directly.
+        false
     }
 
     fn has_css_animations(&self) -> bool {