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

content/renderer/input/input_handler_wrapper.cc

Index: content/renderer/input/input_handler_wrapper.cc
diff --git a/content/renderer/input/input_handler_wrapper.cc b/content/renderer/input/input_handler_wrapper.cc
index b994f09095060872733be792600614c5c8d1c876..d72cc38eb31666a4dd9b24949a41b3f8db00ec35 100644
--- a/content/renderer/input/input_handler_wrapper.cc
+++ b/content/renderer/input/input_handler_wrapper.cc
@@ -90,4 +90,22 @@ void InputHandlerWrapper::DidAnimateForInput() {
   input_handler_manager_->DidAnimateForInput();
 }
 
+void InputHandlerWrapper::GenerateScrollBeginAndSendToMainThread(
+    const blink::WebGestureEvent& update_event) {
+  DCHECK_EQ(update_event.GetType(), blink::WebInputEvent::kGestureScrollUpdate);
+  blink::WebGestureEvent scroll_begin(update_event);
+  scroll_begin.SetType(blink::WebInputEvent::kGestureScrollBegin);
+  scroll_begin.data.scroll_begin.inertial_phase =
+      update_event.data.scroll_update.inertial_phase;
+  scroll_begin.data.scroll_begin.delta_x_hint =
+      update_event.data.scroll_update.delta_x;
+  scroll_begin.data.scroll_begin.delta_y_hint =
+      update_event.data.scroll_update.delta_y;
+  scroll_begin.data.scroll_begin.delta_hint_units =
+      update_event.data.scroll_update.delta_units;
+
+  DispatchNonBlockingEventToMainThread(
+      ui::WebInputEventTraits::Clone(scroll_begin), ui::LatencyInfo());
+}
+
 }  // namespace content