[web-animations-1] Make the timeline argument of the Animation constructor default to null

This fixes w3c#2078.

Author: birtles

web-animations-1/Overview.bs

@@ -3755,7 +3755,7 @@ API by the {{Animation}} interface.
 <pre class='idl'>
 [Exposed=Window,
  Constructor (optional AnimationEffectReadOnly? effect = null,
-              optional AnimationTimeline? timeline)]
+              optional AnimationTimeline? timeline = null)]
 interface Animation : EventTarget {
              attribute DOMString                id;
              attribute AnimationEffectReadOnly? effect;
@@ -3786,10 +3786,7 @@ interface Animation : EventTarget {
     1.  Let <var>animation</var> be a new {{Animation}} object.
     2.  Run the procedure to <a>set the timeline of an animation</a> on
         <var>animation</var> passing <var>timeline</var> as the <var>new
-        timeline</var> or, if a <var>timeline</var> argument is not provided,
-        passing the <a>default document timeline</a> of the {{Document}}
-        <a lt="document associated with a window">associated</a> with the
-        {{Window}} that is the <a>current global object</a>.
+        timeline</var>.
     3.  Run the procedure to <a>set the target effect of an animation</a> on
         <var>animation</var> passing <var>source</var> as the <var>new
         effect</var>.
@@ -3804,11 +3801,8 @@ interface Animation : EventTarget {
     :   <dfn argument
         for="Animation/Animation(effect, timeline)"
         lt="timeline">timeline</dfn>
-    ::  An optional value which, if provided, specifies the <a>timeline</a>
+    ::  An optional value which, if not null, specifies the <a>timeline</a>
         with which to associate the newly-created <a>animation</a>.
-        If not provided, the <a>default document timeline</a> of the
-        {{Document}} <a lt="document associated with a window">associated</a>
-        with the {{Window}} that is the <a>current global object</a> is used.
 
     </div>
 
@@ -5901,6 +5895,10 @@ The following changes have been made since the <a
 *   Added special handling to allow animating the 'offset' property from the
     programming interface using <code>cssOffset</code> to avoid conflict with
     the attribute name used to specify keyframe offsets.
+*   Updated the {{Animation/Animation(effect, timeline)}} constructor to use
+    <code class=es-value>null</code> as the default value when |timeline| is not
+    specified rather than automatically using the appropriate [=default document
+    timeline=].
 *   Updated the procedure to <a>process a keyframes argument</a> to allow
     specifying <code>offset</code>, <code>composite</code> and mulitple
     <code>easing</code> values.