Document the Web Animation interfaces with spec links and TODOs

Source/core/animation/AnimationPlayer.idl

Index: Source/core/animation/AnimationPlayer.idl
diff --git a/Source/core/animation/AnimationPlayer.idl b/Source/core/animation/AnimationPlayer.idl
index ee3da13acadc695881b475db7544123cf8f71fc6..a405eb22a2f74c10f4877e7e1576544b62c4ed68 100644
--- a/Source/core/animation/AnimationPlayer.idl
+++ b/Source/core/animation/AnimationPlayer.idl
@@ -28,6 +28,12 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+// http://www.w3.org/TR/web-animations/#the-animationplayer-interface
+
+// TODO(dstockwell): This interface has been renamed to Animation in the latest
+// spec, which also has several other changes:
+// https://w3c.github.io/web-animations/#the-animation-interface
+
 enum AnimationPlayState { "idle", "pending", "running", "paused", "finished" };
 
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