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..97e4ccc87d16ac7a0e70887e0d2d78e7a957a80f 100644
--- a/public/platform/WebCrypto.h
+++ b/public/platform/WebCrypto.h
@@ -45,6 +45,7 @@
 namespace blink {
 
 class CryptoResult;
+class CryptoResultCancel;
 class WebString;
 
 enum WebCryptoErrorType {
@@ -94,14 +95,15 @@ public:
     BLINK_PLATFORM_EXPORT bool cancelled() const;
 
 #if INSIDE_BLINK
-    BLINK_PLATFORM_EXPORT explicit WebCryptoResult(const PassRefPtrWillBeRawPtr<CryptoResult>&);
+    BLINK_PLATFORM_EXPORT WebCryptoResult(const PassRefPtrWillBeRawPtr<CryptoResult>&, const PassRefPtr<CryptoResultCancel>&);
 #endif
 
 private:
     BLINK_PLATFORM_EXPORT void reset();
     BLINK_PLATFORM_EXPORT void assign(const WebCryptoResult&);
 
-    WebPrivatePtr<CryptoResult> m_impl;
+    WebPrivatePtr<CryptoResult, WebPrivatePtrDestructionCrossThread> m_impl;
+    WebPrivatePtr<CryptoResultCancel, WebPrivatePtrDestructionCrossThread> m_cancel;
 };
 
 class WebCryptoDigestor {