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

LayoutTests/crypto/resources/worker-start-slow-operations.js

+function generateRsaKey()
+{
+    var extractable = false;
+    var usages = ['sign', 'verify'];
+    var algorithm = {name: "RSASSA-PKCS1-v1_5", modulusLength: 512, publicExponent: new Uint8Array([1, 0, 1]), hash: {name: 'sha-1'}};
+
+    return crypto.subtle.generateKey(algorithm, extractable, usages).then(undefined, function(error) {
+        postMessage(error);
+    });
+}
+
+// Start many concurrent operations but don't do anything with the result, or
+// even wait for them to complete.
+for (var i = 0; i < 1000; ++i)
+    generateRsaKey();
+
+// Inform the outer script that the worker started.
+postMessage("Worker started");
+
+close();