Credential manager: Convert strings to URLs early.

LayoutTests/http/tests/credentialmanager/federatedcredential-basics.html

Index: LayoutTests/http/tests/credentialmanager/federatedcredential-basics.html
diff --git a/LayoutTests/http/tests/credentialmanager/federatedcredential-basics.html b/LayoutTests/http/tests/credentialmanager/federatedcredential-basics.html
index 1814a4622266079cd62f195374a2d2345494332d..63ad3b6d6fcde6f7b1923527eecf6d17ed420abc 100644
--- a/LayoutTests/http/tests/credentialmanager/federatedcredential-basics.html
+++ b/LayoutTests/http/tests/credentialmanager/federatedcredential-basics.html
@@ -22,4 +22,16 @@ test(function() {
     assert_equals(credential.avatarURL, 'https://example.com/avatar.png');
     assert_equals(credential.federation, 'https://federation.net/');
 }, 'Interfaces and attributes of FederatedCredential');
+
+test(function() {
+    assert_throws(new SyntaxError(), function () {
+        var credential = new FederatedCredential('id', 'name', '-', 'https://federation.net/');
+    });
+}, 'Construct a FederatedCredential with an invalid avatar URL.');
+
+test(function() {
+    assert_throws(new SyntaxError(), function () {
+        var credential = new FederatedCredential('id', 'name', 'https://example.com/avatar.png', '-');
+    });
+}, 'Construct a FederatedCredential with an invalid federation URL.');
 </script>