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

ui/events/blink/input_handler_proxy_client.h

Index: ui/events/blink/input_handler_proxy_client.h
diff --git a/ui/events/blink/input_handler_proxy_client.h b/ui/events/blink/input_handler_proxy_client.h
index 125d25695c4cab709d1ea75441bbd6e847f4a059..ae9a88b0d8aa9edbf7d01149a1ba08527c6bb4b8 100644
--- a/ui/events/blink/input_handler_proxy_client.h
+++ b/ui/events/blink/input_handler_proxy_client.h
@@ -55,6 +55,11 @@ class InputHandlerProxyClient {
 
   virtual void DidAnimateForInput() = 0;
 
+  // Used to send a GSB to the main thread when the wheel scroll latching is
+  // enabled and the scrolling should switch to the main thread.
+  virtual void GenerateScrollBeginAndSendToMainThread(
+      const blink::WebGestureEvent& update_event) = 0;
+
  protected:
   virtual ~InputHandlerProxyClient() {}
 };