Auto merge of #16381 - BorisChiou:stylo/transition/use_computed_values, r=manishearth

stylo: Bug 1343753 - Use ComputedValues while updating transitions in Gecko

These are interdependent patches of Bug 1343753. We want to use ComputedValues and AnimationValues to start/update/stop the transitions.

---
<!-- 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 Bug 1343753.
- [X] These changes do not require tests because we have tests in Gecko already.

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components/style/gecko/wrapper.rs

@@ -678,6 +678,7 @@ impl<'le> TElement for GeckoElement<'le> {
         let atom_ptr = PseudoElement::ns_atom_or_null_from_opt(pseudo);
         unsafe {
             Gecko_UpdateAnimations(self.0, atom_ptr,
+                                   None,
                                    computed_values_opt,
                                    parent_values_opt,
                                    tasks.bits());