[css-animations] Add elapsedTime to context info for animationstart event

The value of |elapsedTime| is defined for the event so clearly it should
be listed in the context info.

Author: birtles

css-animations/Overview.bs

@@ -930,7 +930,7 @@ Types of <code>AnimationEvent</code></h3>
 			<ul>
 				<li>Bubbles: Yes</li>
 				<li>Cancelable: No</li>
-				<li>Context Info: animationName, pseudoElement</li>
+				<li>Context Info: animationName, elapsedTime, pseudoElement</li>
 			</ul>
 
 		<dt><dfn>animationend</dfn>