Add MediaKeySession.keystatuseschange event

public/platform/WebContentDecryptionModuleSession.h

Index: public/platform/WebContentDecryptionModuleSession.h
diff --git a/public/platform/WebContentDecryptionModuleSession.h b/public/platform/WebContentDecryptionModuleSession.h
index ceca26788fa90f6ae66ba11709fa2b72b2c35a06..fdb7e7bf1f62881aace561323d816f3418da2e64 100644
--- a/public/platform/WebContentDecryptionModuleSession.h
+++ b/public/platform/WebContentDecryptionModuleSession.h
@@ -32,11 +32,13 @@
 #define WebContentDecryptionModuleSession_h
 
 #include "WebCommon.h"
+#include "WebVector.h"
 #include "public/platform/WebContentDecryptionModuleException.h"
 #include "public/platform/WebContentDecryptionModuleResult.h"
 
 namespace blink {
 
+class WebEncryptedMediaKeyInformation;
 class WebString;
 class WebURL;
 
@@ -60,6 +62,13 @@ public:
         // since 01 January, 1970 UTC.
         virtual void expirationChanged(double updatedExpiryTimeInMS) = 0;
 
+        // Called when the set of keys for this session changes or existing keys
+        // change state. |hasAdditionalUsableKey| is set if a key is newly
+        // usable (e.g. new key available, previously expired key has been
+        // renewed, etc.) and the browser should attempt to resume playback
+        // if necessary.
+        virtual void keysStatusesChange(const WebVector<WebEncryptedMediaKeyInformation>&, bool hasAdditionalUsableKey) = 0;
+
     protected:
         virtual ~Client();
     };