[web-animations-1] Convert Animatable to an interface mixin

This fixes #2076.

web-animations-1/Overview.bs

@@ -4831,7 +4831,7 @@ The following methods all accept a set of keyframes as an argument:
 *   the {{KeyframeEffect(target, keyframes, options)}} constructor,
 *   the {{KeyframeEffect/setKeyframes()}} method on the {{KeyframeEffect}}
     interface,
-*   the {{Animatable/animate()}} method of the {{Animatable}} interface.
+*   the {{Animatable/animate()}} method of the {{Animatable}} interface mixin.
 
 This argument may be specified in the one of two forms as illustrated below.
 
@@ -4920,7 +4920,7 @@ This is independent from the <a>timing function</a> that applies to the
 entire <a>iteration duration</a> of the <a>keyframe effect</a> as specified
 using a {{KeyframeEffectOptions}} object (or {{KeyframeAnimationOptions}}
 object when using the {{Animatable/animate()}} method of the {{Animatable}}
-interface).
+interface mixin).
 
 In the following example, the two statements produce different results.
 
@@ -5444,14 +5444,13 @@ enum CompositeOperation {"replace", "add", "accumulate"};
     lt="animation accumulation">accumulated</a> on to the
     <a>underlying value</a>.
 
-<h3 id="the-animatable-interface">The <code>Animatable</code> interface</h3>
+<h3 id="the-animatable-interface-mixin">The <code>Animatable</code> interface mixin</h3>
 
 Objects that may be the target of an {{KeyframeEffectReadOnly}} object implement
-the {{Animatable}} interface.
+the {{Animatable}} interface mixin.
 
 <pre class='idl'>
-[NoInterfaceObject]
-interface Animatable {
+interface mixin Animatable {
     Animation           animate (object? keyframes,
                                  optional (unrestricted double or KeyframeAnimationOptions) options);
     sequence&lt;Animation&gt; getAnimations ();
@@ -5588,9 +5587,9 @@ partial interface Document {
     that have yet to be processed.
 
     Issue(2054): Both this method and {{Animatable/getAnimations()}} on the
-                 {{Animatable}} interface require retaining forwards-filling
-                 <a>animation effects</a> and their <a>animations</a>
-                 such that a document that
+                 {{Animatable}} interface mixin require retaining
+                 forwards-filling <a>animation effects</a> and their
+                 <a>animations</a> such that a document that
                  repeatedly produces forwards-filling animations will consume
                  memory in an unbounded fashion.
                  We may need to revise this definition (previously these methods
@@ -5606,7 +5605,7 @@ Since DOM Elements may be the target of an animation,
 the {{Element}} interface [[!DOM]] is extended as follows:
 
 <pre class="idl">
-Element implements Animatable;
+Element includes Animatable;
 </pre>
 
 This allows the following kind of usage.
@@ -5622,7 +5621,7 @@ Since <a>keyframe effects</a> may also target pseudo-elements, the
 animatable.
 
 <pre class="idl">
-CSSPseudoElement implements Animatable;
+CSSPseudoElement includes Animatable;
 </pre>
 
 <h3 id="the-animationplaybackevent-interface">The <code>AnimationPlaybackEvent</code> interface</h3>
@@ -5910,6 +5909,8 @@ The following changes have been made since the <a
     from <code>Animatable?</code> to <code>(Element or CSSPseudoElement)?</code>
     (<a href="https://github.com/w3c/web-animations/issues/186">#186</a>).
 *   Dropped the <code>SharedKeyframeList</code> interface.
+*   Converted {{Animatable}} from a <code>[NoInterfaceObject]</code> interface
+    to an [=interface mixin=].
 
 The <a
   href="https://github.com/w3c/csswg-drafts/commits/master/web-animations-1">changelog</a>