Mark the ET_MOUSE_MOVED created from a pointer grab as synthesized.

content/browser/renderer_host/render_widget_host_view_event_handler.cc

Index: content/browser/renderer_host/render_widget_host_view_event_handler.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_event_handler.cc b/content/browser/renderer_host/render_widget_host_view_event_handler.cc
index 213ed4ac12129edb76b6884acf2f1e3ff96947dc..a2a9a49313ff8d1d34c864df4f36a82b4ddfd68d 100644
--- a/content/browser/renderer_host/render_widget_host_view_event_handler.cc
+++ b/content/browser/renderer_host/render_widget_host_view_event_handler.cc
@@ -307,9 +307,10 @@ void RenderWidgetHostViewEventHandler::OnMouseEvent(ui::MouseEvent* event) {
   // As the overscroll is handled during scroll events from the trackpad, the
   // RWHVA window is transformed by the overscroll controller. This transform
   // triggers a synthetic mouse-move event to be generated (by the aura
-  // RootWindow). But this event interferes with the overscroll gesture. So,
-  // ignore such synthetic mouse-move events if an overscroll gesture is in
-  // progress.
+  // RootWindow). Also, with a touchscreen, we may get a synthetic mouse-move
+  // caused by a pointer grab. But these events interfere with the overscroll
+  // gesture. So, ignore such synthetic mouse-move events if an overscroll
+  // gesture is in progress.
   OverscrollController* overscroll_controller =
       delegate_->overscroll_controller();
   if (overscroll_controller &&