[webcrypto] Allow crypto operations to be cancelled by the platform implementation.

public/platform/WebCrypto.h

Index: public/platform/WebCrypto.h
diff --git a/public/platform/WebCrypto.h b/public/platform/WebCrypto.h
index 606957792e26db49fcc655d7d76c91db90eea2fd..cc8d15b910b9de146c75f9c506e4fd302b921e22 100644
--- a/public/platform/WebCrypto.h
+++ b/public/platform/WebCrypto.h
@@ -44,6 +44,9 @@ namespace WebCore { class CryptoResult; }
 namespace WTF { template <typename T> class PassRefPtr; }
 #endif
 
+// FIXME: Remove once chromium side rolls.
+#define WEBCRYPTO_RESULT_HAS_CANCELLED 1
+
 namespace blink {
 
 class WebArrayBuffer;
@@ -95,6 +98,10 @@ public:
     BLINK_PLATFORM_EXPORT void completeWithKey(const WebCryptoKey&);
     BLINK_PLATFORM_EXPORT void completeWithKeyPair(const WebCryptoKey& publicKey, const WebCryptoKey& privateKey);
 
+    // Returns true if the underlying operation was cancelled.
+    // This method can be called from any thread.
+    BLINK_PLATFORM_EXPORT bool cancelled() const;
+
 #if INSIDE_BLINK
     BLINK_PLATFORM_EXPORT explicit WebCryptoResult(const WTF::PassRefPtr<WebCore::CryptoResult>&);
 #endif
@@ -146,6 +153,9 @@ public:
     // asynchronously make a copy of the WebCryptoResult and call it from the
     // same thread that started the request.
     //
+    // If the request was cancelled it is not necessary for implementations to
+    // set the result.
+    //
     // -----------------------
     // Threading
     // -----------------------