Fix v01b EME polyfill expiration.

Fix the definition of expiration to be consistent with the details in
w3c/encrypted-media#58 .  In particular, never use infinity.

This means using key status to check if a session is new, rather than
comparing the expiration time to NaN.

Change-Id: Ib1917a34d4dfd0fd29b1c578c6ce60fd996c2698

lib/polyfill/patchedmediakeys_v01b.js

@@ -406,7 +406,7 @@ shaka.polyfill.PatchedMediaKeys.v01b.MediaKeys.prototype.onWebkitKeyMessage_ =
     return;
   }
 
-  var isNew = isNaN(session.expiration);
+  var isNew = session.keyStatuses.getStatus() == undefined;
 
   var event2 = shaka.util.FakeEvent.create({
     type: 'message',
@@ -556,10 +556,6 @@ shaka.polyfill.PatchedMediaKeys.v01b.MediaKeySession.prototype.ready =
     function() {
   shaka.log.debug('v01b.MediaKeySession.ready');
 
-  // There is no expiration info in v0.1b, but we want to signal, at least
-  // internally, that the session is no longer new.  This allows us to set
-  // the messageType attribute of 'message' events.
-  this.expiration = Number.POSITIVE_INFINITY;
   this.updateKeyStatus_('usable');
 
   if (this.updatePromise_) {
@@ -908,6 +904,16 @@ shaka.polyfill.PatchedMediaKeys.v01b.MediaKeyStatusMap.prototype.setStatus =
 };
 
 
+/**
+ * An internal method used by the session to get key status.
+ * @return {string|undefined}
+ */
+shaka.polyfill.PatchedMediaKeys.v01b.MediaKeyStatusMap.prototype.getStatus =
+    function() {
+  return this.status_;
+};
+
+
 /**
  * Array entry 0 is the key, 1 is the value.
  * @return {Iterator.<Array.<!BufferSource|string>>}