Send a GSB to main thread before switching to it in the middle of scrolling.

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 892 matching lines @@
     }
   }
   cc::InputHandlerScrollResult scroll_result =
       input_handler_->ScrollBy(&scroll_state);
 
   if (!scroll_result.did_scroll &&
       input_handler_->ScrollingShouldSwitchtoMainThread() &&
       gesture_event.source_device == blink::kWebGestureDeviceTouchpad &&
       touchpad_and_wheel_scroll_latching_enabled_) {
     gesture_scroll_on_impl_thread_ = false;
+    client_->GenerateScrollBeginAndSendToMainThread(gesture_event);
+
     if (!gesture_pinch_on_impl_thread_)
       return DID_NOT_HANDLE;
   }
 
   HandleOverscroll(scroll_point, scroll_result, true);
 
   if (scroll_elasticity_controller_)
     HandleScrollElasticityOverscroll(gesture_event, scroll_result);
 
   return scroll_result.did_scroll ? DID_HANDLE : DROP_EVENT;
@@ skipping 722 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