NOT FOR LANDING Introduce and use WebMediaSession

Source/modules/mediasession/HTMLMediaElementMediaSession.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/mediasession/HTMLMediaElementMediaSession.h"
 
+#include "Source/core/dom/ExceptionCode.h"
+
 namespace blink {
 
 MediaSession* HTMLMediaElementMediaSession::session(HTMLMediaElement& mediaElement)
 {
     if (HTMLMediaElementMediaSession* supplement = fromIfExists(mediaElement))
         return supplement->m_session.get();
     return nullptr;
 }
 
-void HTMLMediaElementMediaSession::setSession(HTMLMediaElement& mediaElement, MediaSession* session)
+void HTMLMediaElementMediaSession::setSession(HTMLMediaElement& mediaElement, MediaSession* session, ExceptionState& exceptionState)
 {
+    if (mediaElement.networkState() != HTMLMediaElement::NETWORK_EMPTY
+        && mediaElement.networkState() != HTMLMediaElement::NETWORK_NO_SOURCE) {
+        exceptionState.throwDOMException(InvalidStateError, "A media session must be set when before a resource is selected");
+        return;
+    }
+
     from(mediaElement).m_session = session;
 }
 
 const char* HTMLMediaElementMediaSession::supplementName()
 {
     return "HTMLMediaElementMediaSession";
 }
 
 HTMLMediaElementMediaSession& HTMLMediaElementMediaSession::from(HTMLMediaElement& element)
 {
@@ skipping 11 matching lines @@
 }
 
 DEFINE_TRACE(HTMLMediaElementMediaSession)
 {
     visitor->trace(m_session);
     WillBeHeapSupplement<HTMLMediaElement>::trace(visitor);
 }
 
 }