Increase Crane performance test timeout (#268)

See flutter/flutter#64328 (comment)

We'll need to also roll the new gallery version in https://github.com/flutter/flutter/blob/master/dev/devicelab/lib/versions/gallery.dart to have Flutter use this.

Author: liyuqian

test_driver/transitions_perf_test.dart

@@ -248,7 +248,7 @@ void main([List<String> args = const <String>[]]) {
 
       final summary = TimelineSummary.summarize(timeline);
       await summary.writeSummaryToFile('transitions-crane', pretty: true);
-    }, timeout: const Timeout(Duration(seconds: 15)));
+    }, timeout: const Timeout(Duration(seconds: 60)));
 
     test('all demos', () async {
       if (isTestingCraneOnly) return;