Catch unexpected exceptions in sync-xhr test.

With this change, only the expected NetworkError from send() will be
propagated to the feature policy test framework. Other errors (including
NetworkErrors from other calls) will be prefixed with
"UnexpectedException:" and will cause the tests to fail.

Bug: 661283
Change-Id: I75e2fa6526211cc87c65f5f03e926903ae1ba22e
Reviewed-on: https://chromium-review.googlesource.com/874270
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530527}

xhr/xmlhttprequest-sync-default-feature-policy.sub.html

@@ -17,11 +17,15 @@
           try {
             var xhr = new XMLHttpRequest();
             xhr.open("GET", "data:,", false);
-            xhr.send();
-            resolve();
+            try {
+              xhr.send();
+            } catch(e) {
+              reject(e);
+            }
           } catch(e) {
-            reject(e);
+            reject({"name": "UnexpectedException:" + e.name});
           }
+          resolve();
        });
       });
   </script>