linux: Do not fallback to the hardware keycodes for modifers.

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 527 matching lines @@
     MAP3 key4 = {keysym & 0xFFFF, xkey->keycode, keysym_shift & 0xFFFF, 0xFFFF,
                  0};
     const MAP3* p =
         std::lower_bound(map3, map3 + arraysize(map3), key4, MAP3());
     if (p != map3 + arraysize(map3) && p->ch0 == key4.ch0 && p->sc == key4.sc &&
         p->ch1 == key4.ch1)
       return static_cast<KeyboardCode>(p->vk);
   }
 
   keycode = KeyboardCodeFromXKeysym(keysym);
-  if (keycode == VKEY_UNKNOWN)
+  if (keycode == VKEY_UNKNOWN && !IsModifierKey(keysym)) {
+    // Modifier keys should not fall back to the hardware-keycode-based US
+    // layout.  See crbug.com/402320
     keycode = DefaultKeyboardCodeFromHardwareKeycode(xkey->keycode);
+  }
 
   return keycode;
 }
 
 KeyboardCode KeyboardCodeFromXKeysym(unsigned int keysym) {
   // TODO(sad): Have |keysym| go through the X map list?
 
   switch (keysym) {
     case XK_BackSpace:
       return VKEY_BACK;
@@ skipping 787 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