This flash is unavoidable with the current workflow. When the keyboard is hidden, we change back to the default (main) keyset. Then bring the keyboard out with opt-in IME menu, VK is shown before get onhashchange() event. So we will see the main keyboard first and then switch to emoji or hw keyset.

Maybe we need add private API to bring out the keyboard after it getting onhashchange() event and finishing switching keyset. 

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/overlays/chromeos-overlay/+/65abe6c57e9b2353c15f86c5b92a59cc9832afc6

commit 65abe6c57e9b2353c15f86c5b92a59cc9832afc6
Author: azurewei <azurewei@chromium.org>
Date: Tue Dec 27 13:28:43 2016

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/886f64844d90cffa12dbf183eba086becaf819a0

commit 886f64844d90cffa12dbf183eba086becaf819a0
Author: azurewei <azurewei@chromium.org>
Date: Sat Jan 07 08:38:44 2017

Make IME extensions decide when to show the keyboard.

When opening emoji, handwriting or voice input from opt-in IME menu,
InputMethodManager overrides the keyboard url to tell the VK to switch
keyboard and ImeMenuTray call KeyboardController to show the VK.
Since the VK is always back to main keyset when hidden, it will be on
main keyset when shown, and get onhashchanged event to switch to e/h/v
keyset. Thus, has a flash.

To avoid the flash of main keyset, we need to make IME extension decide
when to bring up the VK. So if the KeyboardController is initialized,
and keyboard has been loaded once, it means IME extension could bring
up the VK. Don't show keyboard from ImeMenuTray.

BUG= 667609 
TEST=Verfied on local build.

Review-Url: https://codereview.chromium.org/2603663002
Cr-Commit-Position: refs/heads/master@{#442161}

[modify] https://crrev.com/886f64844d90cffa12dbf183eba086becaf819a0/ash/common/system/chromeos/ime_menu/ime_menu_tray.cc
[modify] https://crrev.com/886f64844d90cffa12dbf183eba086becaf819a0/chrome/browser/chromeos/input_method/input_method_manager_impl.cc
[modify] https://crrev.com/886f64844d90cffa12dbf183eba086becaf819a0/ui/keyboard/keyboard_controller.cc
[modify] https://crrev.com/886f64844d90cffa12dbf183eba086becaf819a0/ui/keyboard/keyboard_controller.h

Verified in M57 9170 clapper