NOT FOR LANDING Introduce and use WebMediaSession

Source/modules/mediasession/HTMLMediaElementMediaSession.cpp

Index: Source/modules/mediasession/HTMLMediaElementMediaSession.cpp
diff --git a/Source/modules/mediasession/HTMLMediaElementMediaSession.cpp b/Source/modules/mediasession/HTMLMediaElementMediaSession.cpp
index ddcde17601ffe90911027ed7909630e6e3bda4cf..3226b4f908a48f5bb1b60c633987209627b941ed 100644
--- a/Source/modules/mediasession/HTMLMediaElementMediaSession.cpp
+++ b/Source/modules/mediasession/HTMLMediaElementMediaSession.cpp
@@ -5,6 +5,8 @@
 #include "config.h"
 #include "modules/mediasession/HTMLMediaElementMediaSession.h"
 
+#include "Source/core/dom/ExceptionCode.h"
+
 namespace blink {
 
 MediaSession* HTMLMediaElementMediaSession::session(HTMLMediaElement& mediaElement)
@@ -14,8 +16,14 @@ MediaSession* HTMLMediaElementMediaSession::session(HTMLMediaElement& mediaEleme
     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;
 }