[web-animations-1] Add timeline to animate() options (#5013)

The options dictionary in `Element.animate` now has an optional nullable timeline
member. This allows timeline to be set via that method in addition to Animation
constructor. The semantic of handling explicit null and implicit/explicit undefined
timelines are the same between the two methods.

Here are some examples of how this may be used:
```
// Create animation with a null timeline.
div.animate({color: 'red' }, { duration: 1000, timeline: null});
```

```
// Create a scroll-linked animation.
div.animate({color: 'red' }, { duration: 1000, timeline: new ScrollTimeline()});
```

Author: majido

web-animations-1/Overview.bs

@@ -5789,6 +5789,7 @@ interface mixin Animatable {
 
 dictionary KeyframeAnimationOptions : KeyframeEffectOptions {
     DOMString id = "";
+    AnimationTimeline? timeline;
 };
 
 dictionary GetAnimationsOptions {
@@ -5813,17 +5814,21 @@ dictionary GetAnimationsOptions {
         If the above procedure causes an exception to be thrown, propagate the
         exception and abort this procedure.
 
+    1.  If <var>options</var> is a {{KeyframeAnimationOptions}} object, let
+        <var>timeline</var> be the <code>timeline</code> member of
+        <var>options</var> or, if <code>timeline</code> member of
+        <var>options</var> is missing, be the <a>default document timeline</a>
+        of the <a>node document</a> of the element on which this method was
+        called.
+
     1.  Construct a new {{Animation}} object, <var>animation</var>, in
         the <a>relevant Realm</a> of <var>target</var> by using the
         same procedure as the {{Animation()}} constructor, passing
-        <var>effect</var> as the argument of the same name, and
-        the <a>default document timeline</a> of the <a>node document</a>
-        of the element on which this method was called as the
-        <var>timeline</var> argument.
+        <var>effect</var> and <var>timeline</var> as arguments of the same name.
 
     1.  If <var>options</var> is a {{KeyframeAnimationOptions}} object,
-        assign the value of the <code>id</code> member of <var>options</var> to
-        <var>animation</var>'s {{Animation/id}} attribute.
+        assign the value of the <code>id</code> member of <var>options</var>
+        to <var>animation</var>'s {{Animation/id}} attribute.
 
     1.  Run the procedure to <a>play an animation</a> for
         <var>animation</var> with the <var>auto-rewind</var> flag set to true.
@@ -5856,10 +5861,8 @@ animation.play();</pre>
         same interpretation as defined for that constructor.
     :   <dfn argument for="Animatable/animate(keyframes, options)"
         lt="options">options</dfn>
-    ::  The timing and animation options for the created {{KeyframeEffect}}.
-        This value is passed to the {{KeyframeEffect(target, keyframes,
-        options)}} constructor as the <var>options</var> parameter and has the
-        same interpretation as defined for that constructor.
+    ::  The timing and animation options for the created {{KeyframeEffect}} and
+        {{Animation}}.
 
     </div>
 
@@ -5910,6 +5913,13 @@ animation.play();</pre>
 ::  The string to assign to the generated {{Animation}}'s {{Animation/id}}
     attribute.
 
+:   <dfn dict-member for=KeyframeAnimationOptions>timeline</dfn>
+::  An optional value which, if present, specifies the <a>timeline</a>
+    with which to associate the newly-created <a>animation</a>.
+    If missing, 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.
+
 :   <dfn dict-member for=GetAnimationsOptions>subtree</dfn>
 ::  If true, indicates that [=animations=] associated with an
     [=animation effect=] whose [=target element=] is a [=descendant=]