[webcrypto] Make WebCryptoAlgorithm "nullable".

public/platform/WebCrypto.h

Index: public/platform/WebCrypto.h
diff --git a/public/platform/WebCrypto.h b/public/platform/WebCrypto.h
index 19462968caf5fbfaf0d5598e1b5c3c1c2750ea38..a76e55ccd11e2b5d138b6b2707da656ba8626403 100644
--- a/public/platform/WebCrypto.h
+++ b/public/platform/WebCrypto.h
@@ -90,12 +90,18 @@ public:
     //
     // The result should be set exactly once, from the same thread which
     // initiated the operation.
+    //
+    //   * WebCryptoAlgorithms parameters are guaranteed to be !isNull(),
+    //     unless noted otherwise.
+    //   * 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(); }
     virtual void sign(const WebCryptoAlgorithm&, const WebCryptoKey&, const unsigned char* data, unsigned dataSize, WebCryptoResult result) { result.completeWithError(); }
     virtual void verifySignature(const WebCryptoAlgorithm&, const WebCryptoKey&, const unsigned char* signature, unsigned signatureSize, const unsigned char* data, unsigned dataSize, WebCryptoResult result) { result.completeWithError(); }
     virtual void digest(const WebCryptoAlgorithm&, const unsigned char* data, unsigned dataSize, WebCryptoResult result) { result.completeWithError(); }
     virtual void generateKey(const WebCryptoAlgorithm&, bool extractable, WebCryptoKeyUsageMask, WebCryptoResult result) { result.completeWithError(); }
+    // The WebCryptoAlgorithm for importKey may be "isNull()" meaning that it
+    // was unspecified by the caller.
     virtual void importKey(WebCryptoKeyFormat, const unsigned char* keyData, unsigned keyDataSize, const WebCryptoAlgorithm&, bool extractable, WebCryptoKeyUsageMask, WebCryptoResult result) { result.completeWithError(); }
     virtual void exportKey(WebCryptoKeyFormat, const WebCryptoKey&, WebCryptoResult result) { result.completeWithError(); }