Add a couple CHECK()s in hopes of helping to track down a crash....

chrome/browser/autocomplete/autocomplete_popup_model.cc

 // Copyright (c) 2006-2008 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/autocomplete/autocomplete_popup_model.h"
 
 #include "base/string_util.h"
 #include "chrome/browser/autocomplete/autocomplete_edit.h"
 #include "chrome/browser/autocomplete/autocomplete_popup_view.h"
 #include "chrome/browser/net/dns_global.h"
@@ skipping 111 matching lines @@
   edit_model_->OnPopupDataChanged(
       reset_to_default ? std::wstring() : match.fill_into_edit,
       !reset_to_default, keyword, is_keyword_hint, match.type);
 
   // Repaint old and new selected lines immediately, so that the edit doesn't
   // appear to update [much] faster than the popup.  We must not update
   // |selected_line_| before calling OnPopupDataChanged() (since the edit may
   // call us back to get data about the old selection), and we must not call
   // UpdateWindow() before updating |selected_line_| (since the paint routine
   // relies on knowing the correct selected line).
+  //
+  // NOTE: We should never reach here with no selected line; the same code that
+  // opened the popup and made it possible to get here should have also set a
+  // selected line.
+  CHECK(selected_line_ != kNoMatch);
   view_->InvalidateLine(selected_line_);
   selected_line_ = line;
   view_->InvalidateLine(selected_line_);
   view_->PaintUpdatesNow();
 }
 
 void AutocompletePopupModel::ResetToDefaultMatch() {
   const AutocompleteResult& result = controller_->result();
   DCHECK(!result.empty());
   SetSelectedLine(result.default_match() - result.begin(), true);
@@ skipping 153 matching lines @@
                                      const NotificationDetails& details) {
   if (inside_synchronous_query_)
     return;
 
   const AutocompleteResult* result =
       Details<const AutocompleteResult>(details).ptr();
   switch (type.value) {
     case NotificationType::AUTOCOMPLETE_CONTROLLER_RESULT_UPDATED: {
       selected_line_ = (result->default_match() == result->end()) ?
           kNoMatch : (result->default_match() - result->begin());
+      // There had better not be a nonempty result set with no default match.
+      CHECK((selected_line_ != kNoMatch) || result->empty());
       // If we're going to trim the window size to no longer include the hovered
       // line, turn hover off.  Practically, this shouldn't happen, but it
       // doesn't hurt to be defensive.
       if ((hovered_line_ != kNoMatch) && (result->size() <= hovered_line_))
         SetHoveredLine(kNoMatch);
 
       view_->UpdatePopupAppearance();
     }
     // FALL THROUGH
 
@@ skipping 26 matching lines @@
       }
       edit_model_->OnPopupDataChanged(inline_autocomplete_text, false, keyword,
           is_keyword_hint, type);
       return;
     }
 
     default:
       NOTREACHED();
   }
 }