Make RWHInputEventRouter manage cross-process scroll bubbling

content/browser/frame_host/render_widget_host_view_child_frame.cc

Index: content/browser/frame_host/render_widget_host_view_child_frame.cc
diff --git a/content/browser/frame_host/render_widget_host_view_child_frame.cc b/content/browser/frame_host/render_widget_host_view_child_frame.cc
index e3a036176bbdbf690e3546a543b0b2c005d3fd63..c70c8e342cfe41990b2eeb243faa00f998ba9b4a 100644
--- a/content/browser/frame_host/render_widget_host_view_child_frame.cc
+++ b/content/browser/frame_host/render_widget_host_view_child_frame.cc
@@ -327,24 +327,22 @@ void RenderWidgetHostViewChildFrame::UnregisterSurfaceNamespaceId() {
   }
 }
 
-void RenderWidgetHostViewChildFrame::WheelEventAck(
-    const blink::WebMouseWheelEvent& event,
-    InputEventAckState ack_result) {
-  if (frame_connector_ &&
-      (ack_result == INPUT_EVENT_ACK_STATE_NOT_CONSUMED ||
-       ack_result == INPUT_EVENT_ACK_STATE_NO_CONSUMER_EXISTS))
-    frame_connector_->BubbleScrollEvent(event);
-}
-
 void RenderWidgetHostViewChildFrame::GestureEventAck(
     const blink::WebGestureEvent& event,
     InputEventAckState ack_result) {
   bool not_consumed = ack_result == INPUT_EVENT_ACK_STATE_NOT_CONSUMED ||
                       ack_result == INPUT_EVENT_ACK_STATE_NO_CONSUMER_EXISTS;
-  // GestureScrollBegin/End are always consumed by the target frame, so we only
-  // forward GestureScrollUpdate.
-  if (frame_connector_ &&
-      event.type == blink::WebInputEvent::GestureScrollUpdate && not_consumed)
+  // GestureScrollBegin is consumed by the target frame and not forwarded,
+  // because we don't know whether we will need to bubble scroll until we
+  // receive a GestureScrollUpdate ACK. GestureScrollUpdate with unused
+  // scroll extent is forwarded for bubbling, while GestureScrollEnd is
+  // always forwarded and handled according to current scroll state in the
+  // RenderWidgetHostInputEventRouter.
+  if (!frame_connector_)
+    return;
+  if ((event.type == blink::WebInputEvent::GestureScrollUpdate &&
+       not_consumed) ||
+      event.type == blink::WebInputEvent::GestureScrollEnd)
     frame_connector_->BubbleScrollEvent(event);
 }