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

ui/events/keycodes/keyboard_code_conversion_x.cc

Index: ui/events/keycodes/keyboard_code_conversion_x.cc
diff --git a/ui/events/keycodes/keyboard_code_conversion_x.cc b/ui/events/keycodes/keyboard_code_conversion_x.cc
index 4311a96f7e5b34d7d41cabf086b4104c738540e2..b42aa8a87a950a8ec7606188249e446980621ad7 100644
--- a/ui/events/keycodes/keyboard_code_conversion_x.cc
+++ b/ui/events/keycodes/keyboard_code_conversion_x.cc
@@ -545,8 +545,11 @@ KeyboardCode KeyboardCodeFromXKeyEvent(const XEvent* xev) {
   }
 
   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;
 }