services/ui/ws/platform_display_default.cc contains this code in PlatformDisplayDefault::DispatchEvent(). It runs after each event is dispatched to the event sink.

  // We want to emulate the WM_CHAR generation behaviour of Windows.
  //
  // On Linux, we've previously inserted characters by having
  // InputMethodAuraLinux take all key down events and send a character event
  // to the TextInputClient. This causes a mismatch in code that has to be
  // shared between Windows and Linux, including blink code. Now that we're
  // trying to have one way of doing things, we need to standardize on and
  // emulate Windows character events.
  //
  // This is equivalent to what we're doing in the current Linux port, but
  // done once instead of done multiple times in different places.
  if (event->type() == ui::ET_KEY_PRESSED) {
    ui::KeyEvent* key_press_event = event->AsKeyEvent();
    ui::KeyEvent char_event(key_press_event->GetCharacter(),
                            key_press_event->key_code(),
                            key_press_event->flags());
    // We don't check that GetCharacter() is equal because changing a key event
    // with an accelerator to a character event can change the character, for
    // example, from 'M' to '^M'.
    DCHECK_EQ(key_press_event->key_code(), char_event.key_code());
    DCHECK_EQ(key_press_event->flags(), char_event.flags());
    SendEventToSink(&char_event);
  }

This results in two ui::ET_KEYPRESSED events for each letter typed. On Chrome OS it causes  issue 706574 , tab moving focus by 2 items. The code isn't necessary on Chrome OS -- the ordinary dispatch works fine. I'm removing it for Chrome OS.

Code was added in https://codereview.chromium.org/1234623004

It's not clear to me or erg@ if this will be needed on Linux Ozone. If not, the code could be removed entirely.