Reliably cancel in-progress CryptoResult(Impl) upon shutdown.

public/platform/WebCrypto.h

Index: public/platform/WebCrypto.h
diff --git a/public/platform/WebCrypto.h b/public/platform/WebCrypto.h
index e0b8174297d704a25e4e3edb665c79ac8db5000b..dcaa3067ac9cd8ee86734f73eaab59b54a4bc750 100644
--- a/public/platform/WebCrypto.h
+++ b/public/platform/WebCrypto.h
@@ -95,13 +95,17 @@ public:
 
 #if INSIDE_BLINK
     BLINK_PLATFORM_EXPORT explicit WebCryptoResult(const PassRefPtrWillBeRawPtr<CryptoResult>&);
+
+    BLINK_PLATFORM_EXPORT void clear();
 #endif
 
 private:
     BLINK_PLATFORM_EXPORT void reset();
     BLINK_PLATFORM_EXPORT void assign(const WebCryptoResult&);
 
-    WebPrivatePtr<CryptoResult> m_impl;
+    int m_cancelled;
+
+    WebPrivatePtr<CryptoResult, WebPrivatePtrDestructionCrossThread> m_impl;
 };
 
 class WebCryptoDigestor {