Update MediaPositionState WebIDL

We make duration a required member of MediaPositionState and add default values for position and playbackRate.
duration is now unrestricted to allow Infinity, we add a special check for NaN.

We update MediaSession.setPositionState to no longer have a default value since MediaPositionState now has a required member.

Fixes w3c#303 and w3c#252.

index.bs

@@ -774,7 +774,7 @@ interface MediaSession {
 
   undefined setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);
 
-  undefined setPositionState(optional MediaPositionState state = {});
+  undefined setPositionState(optional MediaPositionState state);
 
   undefined setMicrophoneActive(boolean active);
 
@@ -879,32 +879,21 @@ interface MediaSession {
 
   <ul>
     <li>
-      If the <var>state</var> is an empty dictionary then clear the <a>position
-      state</a>.
-    </li>
-    <li>
-      If the <a dict-member for="MediaPositionState">duration</a> is not present
-      or its value is null, throw a <a exception>TypeError</a>.
-    </li>
-    <li>
-      If the <a dict-member for="MediaPositionState">duration</a> is negative,
-      throw a <a exception>TypeError</a>.
+      If the optional argument <var>state</var> is not omitted, clear the <a>position
+      state</a> and abort these steps.
     </li>
+
     <li>
-      If the <a dict-member for="MediaPositionState">position</a> is not present
-      or its value is null, set it to zero.
+      If <var>state</var>'s <a dict-member for="MediaPositionState">duration</a> is negative or
+      <code>NaN</code>, throw a <a exception>TypeError</a>.
     </li>
     <li>
-      If the <a dict-member for="MediaPositionState">position</a> is negative or
+      If <var>state</var>'s <a dict-member for="MediaPositionState">position</a> is negative or
       greater than <a dict-member for="MediaPositionState">duration</a>, throw a
       <a exception>TypeError</a>.
     </li>
     <li>
-      If the <a dict-member for="MediaPositionState">playbackRate</a> is not
-      present or its value is null, set it to 1.0.
-    </li>
-    <li>
-      If the <a dict-member for="MediaPositionState">playbackRate</a> is zero
+      If <var>state</var>'s <a dict-member for="MediaPositionState">playbackRate</a> is zero,
       throw a <a exception>TypeError</a>.
     </li>
     <li>
@@ -1204,9 +1193,9 @@ dictionary</h2>
 <pre class="idl">
 
 dictionary MediaPositionState {
-  double duration;
-  double playbackRate;
-  double position;
+  required double duration;
+  double playbackRate = 1.0;
+  double position = 0.0;
 };
 </pre>