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

ui/events/blink/input_handler_proxy.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 305 matching lines @@
   // Note: Other input can race ahead of gesture input as they don't have to go
   // through the queue, but we believe it's OK to do so.
   if (!compositor_event_queue_ ||
       !IsGestureScrollOrFlingOrPinch(event_with_callback->event().GetType())) {
     DispatchSingleInputEvent(std::move(event_with_callback),
                              tick_clock_->NowTicks());
     return;
   }
 
   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());
     if (needs_animate_input)
       input_handler_->SetNeedsAnimateInput();
     return;
   }
 
   // We have to dispatch the event to know whether the gesture sequence will be
   // handled by the compositor or not.
@@ skipping 1309 matching lines @@
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 void InputHandlerProxy::SetTickClockForTesting(
     std::unique_ptr<base::TickClock> tick_clock) {
   tick_clock_ = std::move(tick_clock);
 }
 
 }  // namespace ui