Fix crash when ExecutionContext is torn down before a crypto operation has completed.

LayoutTests/crypto/resources/worker-infinite-loop-generateKey.js

+importScripts('common.js');
+
+function continuouslyGenerateRsaKey()
+{
+    var extractable = false;
+    var usages = ['encrypt', 'decrypt'];
+    // Note that the modulus length is small.
+    var algorithm = {name: "RSAES-PKCS1-v1_5", modulusLength: 512, publicExponent: hexStringToUint8Array("010001")};
+
+    return crypto.subtle.generateKey(algorithm, extractable, usages).then(function(result) {
+        // Infinite recursion intentional!
+        return continuouslyGenerateRsaKey();
+    });
+}
+
+// Starts a Promise which continually generates new RSA keys.
+var unusedPromise = continuouslyGenerateRsaKey();
+
+// Inform the outer script that the worker started.
+postMessage("Worker started");