Invalid press events don't create multiple pointers with the same id.

ui/events/gestures/motion_event_aura.cc

Index: ui/events/gestures/motion_event_aura.cc
diff --git a/ui/events/gestures/motion_event_aura.cc b/ui/events/gestures/motion_event_aura.cc
index 2f532e7f325f6ad0e9f5d211bdba2707ca89834f..174918fedc5a545da380fd2efd3205e36c3e3258 100644
--- a/ui/events/gestures/motion_event_aura.cc
+++ b/ui/events/gestures/motion_event_aura.cc
@@ -82,12 +82,17 @@ bool MotionEventAura::OnTouch(const TouchEvent& touch) {
   bool pointer_id_is_active = index != -1;
 
   if (touch.type() == ET_TOUCH_PRESSED && pointer_id_is_active) {
-    // Ignore touch press events if we already believe the pointer is down.
-
-    // TODO(tdresser): this should return false (or NOTREACHED());
-    // however, there is at least one case where we need to allow a
-    // touch press from a currently used touch id. See
+    // TODO(tdresser): This should return false (or NOTREACHED()), and
+    // ignore the touch; however, there is at least one case where we
+    // need to allow a touch press from a currently used touch id. See
     // crbug.com/446852 for details.
+
+    // Cancel the existing touch, before handling the touch press.
+    TouchEvent cancel(ET_TOUCH_CANCELLED, touch.location(), touch.touch_id(),
+                      touch.time_stamp());
+    OnTouch(cancel);
+    CleanupRemovedTouchPoints(cancel);
+    DCHECK_EQ(-1, FindPointerIndexOfId(touch.touch_id()));
   } else if (touch.type() != ET_TOUCH_PRESSED && !pointer_id_is_active) {
     // We could have an active touch stream transfered to us, resulting in touch
     // move or touch up events without associated touch down events. Ignore