Async Wheel Event with only the first one cancellable behind a flag.

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 6ff1983d7139c16be4b7604726a8c80db40dea7f..7d78d0b668009ef2c329048123734fcccc7c930e 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -666,6 +666,24 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
     CancelCurrentFling();
 
   InputHandlerProxy::EventDisposition result = DROP_EVENT;
+
+  if (wheel_event.dispatch_type == WebInputEvent::kEventNonBlocking) {
+    // The first wheel event in the sequence should be cancellable.
+    DCHECK(wheel_event.phase != WebMouseWheelEvent::kPhaseBegan);
+
+    DCHECK(mouse_wheel_result_ != kEventDispositionUndefined);
+    result = static_cast<EventDisposition>(mouse_wheel_result_);
+
+    if (wheel_event.phase == WebMouseWheelEvent::kPhaseEnded ||
+        wheel_event.phase == WebMouseWheelEvent::kPhaseCancelled ||
+        wheel_event.momentum_phase == WebMouseWheelEvent::kPhaseEnded ||
+        wheel_event.momentum_phase == WebMouseWheelEvent::kPhaseCancelled) {
+      mouse_wheel_result_ = kEventDispositionUndefined;
+    }
+
+    return result;
+  }
+
   cc::EventListenerProperties properties =
       input_handler_->GetEventListenerProperties(
           cc::EventListenerClass::kMouseWheel);