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

LayoutTests/crypto/exportKey-badParameters.html

Index: LayoutTests/crypto/exportKey-badParameters.html
diff --git a/LayoutTests/crypto/exportKey-badParameters.html b/LayoutTests/crypto/exportKey-badParameters.html
index 61244af2f04e1d586ba1e93a3c36deef573dba24..2ea3e0d092422c4aa98711fc68a0a50dbbf0b752 100644
--- a/LayoutTests/crypto/exportKey-badParameters.html
+++ b/LayoutTests/crypto/exportKey-badParameters.html
@@ -27,14 +27,12 @@ Promise.resolve(null).then(function(result) {
     // null is not a valid Key.
     return crypto.subtle.exportKey('raw', null);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // 3 is not a valid Key.
     return crypto.subtle.exportKey('raw', 3);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     return importAesKey();
 }).then(function(result) {
@@ -43,20 +41,17 @@ Promise.resolve(null).then(function(result) {
     // Invalid export format
     return crypto.subtle.exportKey(3, key);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // Invalid export format
     return crypto.subtle.exportKey(null, key);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 
     // Invalid export format
     return crypto.subtle.exportKey('invalid', key);
 }).then(failAndFinishJSTest, function(result) {
-    error = result;
-    shouldBeNull("error");
+    logError(result);
 }).then(finishJSTest, failAndFinishJSTest);
 
 </script>