Added extra DCHECK() to eliminate possible NULL deref.

chrome/browser/ui/views/tabs/tab_drag_controller.cc

 // Copyright (c) 2012 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 "chrome/browser/ui/views/tabs/tab_drag_controller.h"
 
 #include <math.h>
 #include <set>
 
 #include "base/callback.h"
@@ skipping 1592 matching lines @@
       }
     }
   }
   if (!has_one_valid_tab)
     return;
 
   model->SetSelectionFromModel(selection_model);
 }
 
 void TabDragController::RevertDragAt(size_t drag_index) {
-  DCHECK(started_drag_);
+  DCHECK(started_drag_ && source_tabstrip_);

[James Hawkins, 2012/07/26 00:10:50]

nit: Don't put two conditions in one DCHECK, otherwise it's harder to see which
value is causing the failure.

 
   TabDragData* data = &(drag_data_[drag_index]);
   if (attached_tabstrip_) {
     int index =
         GetModel(attached_tabstrip_)->GetIndexOfTabContents(data->contents);
     if (attached_tabstrip_ != source_tabstrip_) {
       // The Tab was inserted into another TabStrip. We need to put it back
       // into the original one.
       GetModel(attached_tabstrip_)->DetachTabContentsAt(index);
       // TODO(beng): (Cleanup) seems like we should use Attach() for this
@@ skipping 298 matching lines @@
     gfx::Point touch_point;
     bool got_touch_point = widget_window->GetRootWindow()->
         gesture_recognizer()->GetLastTouchPointForTarget(widget_window,
                                                          &touch_point);
     DCHECK(got_touch_point);
     return touch_point;
   }
 #endif
   return gfx::Screen::GetCursorScreenPoint();
 }