Fix crash happens when VK tray is clicked after VK is disabled.

Bug: 749989
Test: manually
Change-Id: I547151f5950e07e3669d0f13ce41b951cd354f4f
Reviewed-on: https://chromium-review.googlesource.com/593434
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Keigo Oka <oka@chromium.org>
Commit-Queue: Keigo Oka <oka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492052}
void KeyboardUI::RemoveObserver(KeyboardUIObserver* observer) {
  observers_.RemoveObserver(observer);
}

void KeyboardUI::RemoveObserver(KeyboardUIObserver* observer) {
  observers_.RemoveObserver(observer);
}
