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

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 911147a1f3711d6f2c3194c06dbe65a532754199..d952c21a0b1344dc6dfa4c093e78df3bce1d89dd 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -910,6 +910,8 @@ InputHandlerProxy::HandleGestureScrollUpdate(
       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;
   }