Disable backspace until I have fixed it in IME mode.

chrome/browser/global_keyboard_shortcuts_mac.mm

 // Copyright (c) 2009 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 <Carbon/Carbon.h>
 
 #include "chrome/browser/global_keyboard_shortcuts_mac.h"
 
 #include "base/basictypes.h"
 #include "chrome/app/chrome_dll_resource.h"
@@ skipping 22 matching lines @@
   *num_entries = arraysize(keyboard_shortcuts);
 
   return keyboard_shortcuts;
 }
 
 const KeyboardShortcutData* GetBrowserKeyboardShortcutTable
     (size_t* num_entries) {
   static const KeyboardShortcutData keyboard_shortcuts[] = {
     {true,  false, false, kVK_LeftArrow,     IDC_BACK},
     {true,  false, false, kVK_RightArrow,    IDC_FORWARD},
-    {false, false, false, kVK_Delete,        IDC_BACK},
-    {false, true,  false, kVK_Delete,        IDC_FORWARD},
   };
 
   *num_entries = arraysize(keyboard_shortcuts);
 
   return keyboard_shortcuts;
 }
 
 static int CommandForKeyboardShortcut(
     const KeyboardShortcutData* (*get_keyboard_shortcut_table)(size_t*),
     bool command_key, bool shift_key, bool cntrl_key, int vkey_code) {
@@ skipping 23 matching lines @@
                                     command_key, shift_key,
                                     cntrl_key, vkey_code);
 }
 
 int CommandForBrowserKeyboardShortcut(
     bool command_key, bool shift_key, bool cntrl_key, int vkey_code) {
   return CommandForKeyboardShortcut(GetBrowserKeyboardShortcutTable,
                                     command_key, shift_key,
                                     cntrl_key, vkey_code);
 }