WebCrypto: Add interface for exportKey().

public/platform/WebCrypto.h

Index: public/platform/WebCrypto.h
diff --git a/public/platform/WebCrypto.h b/public/platform/WebCrypto.h
index 2d8dc5f957e4a712229509b6d664accac0da7e08..70e10f1dfaa8bb3190addf9d6b53e375e132127e 100644
--- a/public/platform/WebCrypto.h
+++ b/public/platform/WebCrypto.h
@@ -97,6 +97,7 @@ public:
     virtual void digest(const WebCryptoAlgorithm&, const unsigned char* data, size_t data_size, WebCryptoResult) { WEBKIT_ASSERT_NOT_REACHED(); }
     virtual void generateKey(const WebCryptoAlgorithm&, bool extractable, WebCryptoKeyUsageMask, WebCryptoResult) { WEBKIT_ASSERT_NOT_REACHED(); }
     virtual void importKey(WebCryptoKeyFormat, const unsigned char* keyData, size_t keyDataSize, const WebCryptoAlgorithm&, bool extractable, WebCryptoKeyUsageMask, WebCryptoResult) { WEBKIT_ASSERT_NOT_REACHED(); }
+    virtual void exportKey(WebCryptoKeyFormat, const WebCryptoKey&, WebCryptoResult) { WEBKIT_ASSERT_NOT_REACHED(); }
 
 protected:
     virtual ~WebCrypto() { }