LibWeb: Make sure finished animations are not restarted

This commit makes the StyleComputer avoid restarting finished animations
(e.g. animations with finite iteration counts that have run to
completion).
As a bonus, it also disables the animation timer when all animations
have finished running.

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -205,6 +205,7 @@ private:
     };
 
     mutable HashMap<AnimationKey, NonnullOwnPtr<Animation>> m_active_animations;
+    mutable HashTable<AnimationKey> m_finished_animations;
     mutable RefPtr<Platform::Timer> m_animation_driver_timer;
 };