back gesture: disable back gesture for PEN and ERASER events.

Bug: 1063136
Change-Id: I5c0136e77e9b0859aff91bb726b09d0b21ed7360
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125298Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754549}

ash/wm/gestures/back_gesture/back_gesture_event_handler.cc

@@ -162,6 +162,15 @@ void BackGestureEventHandler::OnDisplayMetricsChanged(
 void BackGestureEventHandler::OnGestureEvent(ui::GestureEvent* event) {}
 
 void BackGestureEventHandler::OnTouchEvent(ui::TouchEvent* event) {
+  // Do not handle PEN and ERASER events for back gesture. PEN events can come
+  // from stylus device.
+  if (event->pointer_details().pointer_type ==
+          ui::EventPointerType::POINTER_TYPE_PEN ||
+      event->pointer_details().pointer_type ==
+          ui::EventPointerType::POINTER_TYPE_ERASER) {
+    return;
+  }
+
   if (first_touch_id_ == ui::kPointerIdUnknown)
     first_touch_id_ = event->pointer_details().id;