Merge pull request #122 from Unity-Technologies/zxw/fix_scroll_assertion_error

fix scroll assertion error

com.unity.uiwidgets/Runtime/gestures/binding.cs

                 if (evt is PointerDownEvent) {
                     _hitTests[evt.pointer] = hitTestResult;
                 }
-
-                _hitTests[evt.pointer] = hitTestResult;
                 D.assert(() => {
                     if (D.debugPrintHitTestResults) {
                         Debug.LogFormat("{0}: {1}", evt, hitTestResult);