Credential manager: Convert strings to URLs early.

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

Index: LayoutTests/http/tests/credentialmanager/localcredential-basics.html
diff --git a/LayoutTests/http/tests/credentialmanager/localcredential-basics.html b/LayoutTests/http/tests/credentialmanager/localcredential-basics.html
index 38026638f32f69fef08fc2d5a8202213644c1c28..31675ab3c8447df2f800f06fc4c6f957411f13f2 100644
--- a/LayoutTests/http/tests/credentialmanager/localcredential-basics.html
+++ b/LayoutTests/http/tests/credentialmanager/localcredential-basics.html
@@ -19,4 +19,13 @@ test(function() {
     assert_equals(credential.avatarURL, 'https://example.com/avatar.png');
     assert_equals(credential.password, 'pencil');
 }, 'Interfaces and attributes of LocalCredential');
+
+test(function() {
+    try {
+        var credential = new LocalCredential('id', 'name', '-', 'pencil');
+        assert_unreached("Creating a credential with an invalid URL should throw a SyntaxError.");
+    } catch (e) {
+        assert_equals(e.name, "SyntaxError");
+    }
+}, 'Invalid avatar URL throws.');
 </script>