Mark the ET_MOUSE_MOVED created from a pointer grab as synthesized.

content/browser/renderer_host/render_widget_host_view_event_handler.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/render_widget_host_view_event_handler.h"
 
 #include "base/metrics/user_metrics.h"
 #include "base/metrics/user_metrics_action.h"
 #include "content/browser/renderer_host/input/touch_selection_controller_client_aura.h"
 #include "content/browser/renderer_host/overscroll_controller.h"
@@ skipping 289 matching lines @@
   // event->handled to true (http://crbug.com/577983).
 
   if (mouse_locked_) {
     HandleMouseEventWhileLocked(event);
     return;
   }
 
   // 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 &&
       overscroll_controller->overscroll_mode() != OVERSCROLL_NONE &&
       event->flags() & ui::EF_IS_SYNTHESIZED &&
       (event->type() == ui::ET_MOUSE_ENTERED ||
        event->type() == ui::ET_MOUSE_EXITED ||
        event->type() == ui::ET_MOUSE_MOVED)) {
     event->StopPropagation();
     return;
@@ skipping 580 matching lines @@
   host_->ForwardWheelEventWithLatencyInfo(event, latency);
 }
 
 void RenderWidgetHostViewEventHandler::ProcessTouchEvent(
     const blink::WebTouchEvent& event,
     const ui::LatencyInfo& latency) {
   host_->ForwardTouchEventWithLatencyInfo(event, latency);
 }
 
 }  // namespace content