Missing source pointer down events are handled.

ui/events/gesture_detection/gesture_provider.cc

 // Copyright 2014 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 "ui/events/gesture_detection/gesture_provider.h"
 
 #include <stddef.h>
 
 #include <cmath>
 
@@ skipping 674 matching lines @@
       const MotionEvent& secondary_pointer_down) {
     // If there are more than two down pointers, tapping is not possible,
     // so Slop region is not deducted.
     DCHECK(ev2.GetPointerCount() < 3);
 
     gfx::Vector2dF delta(0, 0);
     for (size_t i = 0; i < ev2.GetPointerCount(); i++) {
       const int pointer_id = ev2.GetPointerId(i);
       const MotionEvent* source_pointer_down_event =
           gesture_detector_.GetSourcePointerDownEvent(
-              ev1, secondary_pointer_down, pointer_id);
-      DCHECK(source_pointer_down_event);
+              ev1, &secondary_pointer_down, pointer_id);
+
       if (!source_pointer_down_event)
         continue;
       int source_index =
           source_pointer_down_event->FindPointerIndexOfId(pointer_id);
       DCHECK_GE(source_index, 0);
       if (source_index < 0)
         continue;
       float dx = source_pointer_down_event->GetX(source_index) - ev2.GetX(i);
       float dy = source_pointer_down_event->GetY(source_index) - ev2.GetY(i);
       delta += SubtractSlopRegion(dx, dy);
@@ skipping 205 matching lines @@
   // null'ing of the listener until the sequence has ended.
   if (current_down_event_)
     return;
 
   const bool double_tap_enabled =
       double_tap_support_for_page_ && double_tap_support_for_platform_;
   gesture_listener_->SetDoubleTapEnabled(double_tap_enabled);
 }
 
 }  //  namespace ui