[VSync Queue] Plug touch ack to gesture events and flush vsync queue if necessary

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index c797a6852415cc092bdd5990de4fefecd063ddb6..0f26dd9f7641c7c7fd8e21dd283b3544c2f515bc 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -324,6 +324,17 @@ void InputHandlerProxy::HandleInputEventWithLatencyInfo(
   }
 
   if (has_ongoing_compositor_scroll_fling_pinch_) {
+    const auto& gesture_event = ToWebGestureEvent(event_with_callback->event());
+    if (gesture_event.source_device == blink::kWebGestureDeviceTouchscreen &&
+        gesture_event.is_source_touch_event_set_non_blocking) {
+      // Dispatch immediately to avoid regression in
+      // |smoothness.tough_scrolling_cases:first_gesture_scroll_update_latency|.
+      compositor_event_queue_->Queue(std::move(event_with_callback),
+                                     tick_clock_->NowTicks());
+      DispatchQueuedInputEvents();
+      return;
+    }
+
     bool needs_animate_input = compositor_event_queue_->empty();
     compositor_event_queue_->Queue(std::move(event_with_callback),
                                    tick_clock_->NowTicks());