Add MediaKeySession.expiration attribute.

public/platform/WebContentDecryptionModuleSession.h

Index: public/platform/WebContentDecryptionModuleSession.h
diff --git a/public/platform/WebContentDecryptionModuleSession.h b/public/platform/WebContentDecryptionModuleSession.h
index ab08884ff61732ada3719cee68ebb52a8e538f83..d12f419ec8f0bb3f502eb0d9f77bfb37811f1ed2 100644
--- a/public/platform/WebContentDecryptionModuleSession.h
+++ b/public/platform/WebContentDecryptionModuleSession.h
@@ -56,6 +56,10 @@ public:
         virtual void error(MediaKeyErrorCode, unsigned long systemCode) = 0;
         virtual void error(WebContentDecryptionModuleException, unsigned long systemCode, const WebString& message) = 0;
 
+        // Called when the expiration time for the session changes. |time| is
+        // specified as the number of milliseconds since 01 January, 1970 UTC.
+        virtual void expirationChanged(double updatedExpiryTimeInMS) = 0;
+
     protected:
         virtual ~Client();
     };