Add MediaKeySession.expiration attribute.

Source/modules/encryptedmedia/MediaKeySession.h

Index: Source/modules/encryptedmedia/MediaKeySession.h
diff --git a/Source/modules/encryptedmedia/MediaKeySession.h b/Source/modules/encryptedmedia/MediaKeySession.h
index a14b131bfe74c5e84dd1582d2382cf1822492daf..eae0e31cbf73065e56e4a85fab18da37fe8de0a6 100644
--- a/Source/modules/encryptedmedia/MediaKeySession.h
+++ b/Source/modules/encryptedmedia/MediaKeySession.h
@@ -70,6 +70,7 @@ public:
 
     const String& keySystem() const { return m_keySystem; }
     String sessionId() const;
+    double expiration() const { return m_expiration; }
     ScriptPromise closed(ScriptState*);
 
     ScriptPromise generateRequest(ScriptState*, const String& initDataType, ArrayBuffer* initData);
@@ -126,6 +127,7 @@ private:
 
     // Session properties.
     String m_sessionType;
+    double m_expiration;
 
     // Session states.
     bool m_isUninitialized;