[webcrypto] Make WebCryptoKey nullable.

public/platform/WebCrypto.h

Index: public/platform/WebCrypto.h
diff --git a/public/platform/WebCrypto.h b/public/platform/WebCrypto.h
index a76e55ccd11e2b5d138b6b2707da656ba8626403..5f538682f03b3417089020f187835c8505a0523e 100644
--- a/public/platform/WebCrypto.h
+++ b/public/platform/WebCrypto.h
@@ -93,6 +93,7 @@ public:
     //
     //   * WebCryptoAlgorithms parameters are guaranteed to be !isNull(),
     //     unless noted otherwise.
+    //   * WebCryptoKey parameters are guaranteeed to be !isNull().
     //   * const unsigned char* data buffers are not valid after return.
     virtual void encrypt(const WebCryptoAlgorithm&, const WebCryptoKey&, const unsigned char* data, unsigned dataSize, WebCryptoResult result) { result.completeWithError(); }
     virtual void decrypt(const WebCryptoAlgorithm&, const WebCryptoKey&, const unsigned char* data, unsigned dataSize, WebCryptoResult result) { result.completeWithError(); }