[webcrypto] Match the error handling defined by the spec.

Source/modules/crypto/Key.h

Index: Source/modules/crypto/Key.h
diff --git a/Source/modules/crypto/Key.h b/Source/modules/crypto/Key.h
index aa6bad09ffb7d10b00bd65ece7dd254a97d4e154..598ab9fbc620c7d5fc2f5a20740de57912834916 100644
--- a/Source/modules/crypto/Key.h
+++ b/Source/modules/crypto/Key.h
@@ -57,7 +57,9 @@ public:
 
     const blink::WebCryptoKey& key() const { return m_key; }
 
-    bool canBeUsedForAlgorithm(const blink::WebCryptoAlgorithm&, AlgorithmOperation, ExceptionState&) const;
+    // If the key cannot be used with the indicated algorithm, returns false
+    // and fills the provided String with error details.
+    bool canBeUsedForAlgorithm(const blink::WebCryptoAlgorithm&, AlgorithmOperation, String&) const;
 
     static bool parseFormat(const String&, blink::WebCryptoKeyFormat&, ExceptionState&);
     static bool parseUsageMask(const Vector<String>&, blink::WebCryptoKeyUsageMask&, ExceptionState&);