Revert of Web Animations: Remove AnimationPlayer discarding logic

Source/core/animation/AnimationPlayer.h

Index: Source/core/animation/AnimationPlayer.h
diff --git a/Source/core/animation/AnimationPlayer.h b/Source/core/animation/AnimationPlayer.h
index a5c414a2718332488c7e51b5763c18b7a9cc4789..dc1739552c2365aac6592bf6939849cd1bc1ef09 100644
--- a/Source/core/animation/AnimationPlayer.h
+++ b/Source/core/animation/AnimationPlayer.h
@@ -162,6 +162,12 @@
         return player1->sequenceNumber() < player2->sequenceNumber();
     }
 
+#if !ENABLE(OILPAN)
+    // Checks if the AnimationStack is the last reference holder to the Player.
+    // This won't be needed when AnimationPlayer is moved to Oilpan.
+    bool canFree() const;
+#endif
+
     virtual bool addEventListener(const AtomicString& eventType, PassRefPtr<EventListener>, bool useCapture = false) override;
 
     DECLARE_VIRTUAL_TRACE();