Add Alias and Checks to TabDragController to isolate crash

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/auto_reset.h"
 #include "base/callback.h"
+#include "base/debug/alias.h"
 #include "base/i18n/rtl.h"
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser_list.h"
 #include "chrome/browser/ui/browser_window.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/browser/ui/tabs/tab_strip_model_delegate.h"
 #include "chrome/browser/ui/views/frame/browser_view.h"
@@ skipping 830 matching lines @@
   if (!browser_view ||
       !browser_view->browser()->SupportsWindowFeature(
           Browser::FEATURE_TABSTRIP))
     return NULL;
 
   TabStrip* other_tabstrip = browser_view->tabstrip();
   TabStrip* tab_strip =
       attached_tabstrip_ ? attached_tabstrip_ : source_tabstrip_;
   DCHECK(tab_strip);
 
+  CHECK(instance_);
+  if (!other_tabstrip->controller() || !tab_strip->controller()) {
+    base::debug::Alias(attached_tabstrip_);
+    base::debug::Alias(source_tabstrip_);
+    base::debug::Alias(other_tabstrip);
+    CHECK(other_tabstrip->controller());
+    CHECK(tab_strip->controller());
+  }
+
   return other_tabstrip->controller()->IsCompatibleWith(tab_strip) ?
       other_tabstrip : NULL;
 }
 
 bool TabDragController::DoesTabStripContain(
     TabStrip* tabstrip,
     const gfx::Point& point_in_screen) const {
   // Make sure the specified screen point is actually within the bounds of the
   // specified tabstrip...
   gfx::Rect tabstrip_bounds = GetViewScreenBounds(tabstrip);
@@ skipping 925 matching lines @@
   // window which was used for dragging is not hidden once all of its tabs are
   // attached to another browser window in DragBrowserToNewTabStrip().
   // TODO(pkotwicz): Fix this properly (crbug.com/358482)
   for (auto* browser : *BrowserList::GetInstance()) {
     if (browser->tab_strip_model()->empty())
       exclude.insert(browser->window()->GetNativeWindow());
   }
 #endif
   return GetLocalProcessWindowAtPoint(screen_point, exclude);
 }