[webcrypto] Reject failed operations with a DOMException rather than null.

LayoutTests/crypto/aes-kw-generateKey-parseAlgorithm-failures.html

Index: LayoutTests/crypto/aes-kw-generateKey-parseAlgorithm-failures.html
diff --git a/LayoutTests/crypto/aes-kw-generateKey-parseAlgorithm-failures.html b/LayoutTests/crypto/aes-kw-generateKey-parseAlgorithm-failures.html
index a1ebe38039a34a7b40daff6ae8b0fcb5f5c40cd5..9c17fada6e3c1dff36e8232720acb1d705217473 100644
--- a/LayoutTests/crypto/aes-kw-generateKey-parseAlgorithm-failures.html
+++ b/LayoutTests/crypto/aes-kw-generateKey-parseAlgorithm-failures.html
@@ -18,23 +18,19 @@ keyUsages = ['wrapKey', 'unwrapKey'];
 Promise.resolve(null).then(function() {
     return crypto.subtle.generateKey({ name: 'aes-kw' }, extractable, keyUsages);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     return crypto.subtle.generateKey({ name: 'aes-kw', length: 70000 }, extractable, keyUsages);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     return crypto.subtle.generateKey({ name: 'aes-kw', length: -3 }, extractable, keyUsages);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     return crypto.subtle.generateKey({ name: 'aes-kw', length: -Infinity }, extractable, keyUsages);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 }).then(finishJSTest, failAndFinishJSTest);
 
 </script>