[webcrypto] Make it safe to delete WebCryptoResult from any thread.

Source/platform/CryptoResult.h

Index: Source/platform/CryptoResult.h
diff --git a/Source/platform/CryptoResult.h b/Source/platform/CryptoResult.h
index 393549aae1d7b40ec78fcb95988bad348ea0d72d..4c4e6d22a1b27a5090f103b3c8255b7abb580f58 100644
--- a/Source/platform/CryptoResult.h
+++ b/Source/platform/CryptoResult.h
@@ -32,12 +32,12 @@
 #define CryptoResult_h
 
 #include "public/platform/WebCrypto.h"
-#include "wtf/RefCounted.h"
+#include "wtf/ThreadSafeRefCounted.h"
 
 namespace WebCore {
 
 // Receives notification of completion of the crypto operation.
-class CryptoResult : public RefCounted<CryptoResult> {
+class CryptoResult : public ThreadSafeRefCounted<CryptoResult> {
 public:
     virtual ~CryptoResult() { }