If keysym is ASCII control or space keys, directly map to keycode.

ui/events/keycodes/keyboard_code_conversion_x.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 "ui/events/keycodes/keyboard_code_conversion_x.h"
 
 #include <algorithm>
 
 #define XK_3270  // for XK_3270_BackTab
 #include <X11/XF86keysym.h>
@@ skipping 474 matching lines @@
   } else {
     xkeyevent.xkey = xev->xkey;
   }
   KeyboardCode keycode = VKEY_UNKNOWN;
   XKeyEvent* xkey = &xkeyevent.xkey;
   // XLookupKeysym does not take into consideration the state of the lock/shift
   // etc. keys. So it is necessary to use XLookupString instead.
   XLookupString(xkey, NULL, 0, &keysym, NULL);
   if (IsKeypadKey(keysym) || IsPrivateKeypadKey(keysym) ||
       IsCursorKey(keysym) || IsPFKey(keysym) || IsFunctionKey(keysym) ||
-      IsModifierKey(keysym)) {
+      IsModifierKey(keysym) || keysym == XK_Return || keysym == XK_Tab ||
+      keysym == XK_BackSpace || keysym == XK_space) {

[Yuki, 2014/10/23 06:33:44]

Do we want to support other control characters here?
For example, vertical tab, form feed, escape, etc.

We may want to treat all control characters in ASCII (0x01 to 0x1F) + space
(0x20) here.  You may want to add such a helper function, say
IsAsciiControlKey(keysym).  Though, note that space is not a control character.

[Shu Chen, 2014/10/23 07:00:17]

Done. Thanks for the suggestion.

     return KeyboardCodeFromXKeysym(keysym);
   }
 
   // If |xkey| has modifiers set, other than NumLock, then determine the
   // un-modified KeySym and use that to map, so that e.g. Ctrl+D correctly
   // generates VKEY_D.
   if (xkey->state & 0xFF & ~Mod2Mask) {
     xkey->state &= (~0xFF | Mod2Mask);
     XLookupString(xkey, NULL, 0, &keysym, NULL);
   }
@@ skipping 852 matching lines @@
   // alone does not map to XK_less; XKeysymToKeycode() returns KEY_102ND
   // (the '<>' key between Shift and Z on 105-key keyboards) which does.
   //
   // crbug.com/386066 and crbug.com/390263 are examples of problems
   // associated with this.
   //
   return XKeysymToKeycode(display, XKeysymForWindowsKeyCode(key_code, false));
 }
 
 }  // namespace ui