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

Source/modules/crypto/CryptoResultImpl.h

Index: Source/modules/crypto/CryptoResultImpl.h
diff --git a/Source/modules/crypto/CryptoResultImpl.h b/Source/modules/crypto/CryptoResultImpl.h
index aa5428b0c0f870d61787d941a7bcfe6fc555eb67..8e0ffe7aca43ea738b5e6403f94e3f788ba86af0 100644
--- a/Source/modules/crypto/CryptoResultImpl.h
+++ b/Source/modules/crypto/CryptoResultImpl.h
@@ -43,6 +43,7 @@
 namespace WebCore {
 
 class ScriptPromiseResolver;
+class ScriptState;
 
 // Wrapper around a Promise to notify completion of the crypto operation.
 // Platform cannot know about Promises which are declared in bindings.
@@ -53,6 +54,7 @@ public:
     static PassRefPtr<CryptoResultImpl> create(ScriptPromise);
 
     virtual void completeWithError() OVERRIDE;
+    virtual void completeWithError(const blink::WebString&) OVERRIDE;
     virtual void completeWithBuffer(const blink::WebArrayBuffer&) OVERRIDE;
     virtual void completeWithBoolean(bool) OVERRIDE;
     virtual void completeWithKey(const blink::WebCryptoKey&) OVERRIDE;