[eme] Handle multiple calls to MediaKeySession.close()

media/base/content_decryption_module.h

Index: media/base/content_decryption_module.h
diff --git a/media/base/content_decryption_module.h b/media/base/content_decryption_module.h
index e8af761dcf9efdd51a6f378a9f2bcc0317b94926..3fe3eb6f2c4cfc2dd465d5c70a0c37db6e59c3eb 100644
--- a/media/base/content_decryption_module.h
+++ b/media/base/content_decryption_module.h
@@ -125,6 +125,8 @@ class MEDIA_EXPORT ContentDecryptionModule
   // reject the |promise| when the call has been processed. This may be before
   // the session is closed. Once the session is closed, a SessionClosedCB must
   // also be called.
+  // Note that the EME spec executes the close() action asynchronously, so
+  // CloseSession() may be called multiple times on the same session.
   virtual void CloseSession(const std::string& session_id,
                             std::unique_ptr<SimpleCdmPromise> promise) = 0;