Add --keyboard-usability-test flag and always show keyboard unless manually hide

ui/keyboard/keyboard_switches.cc

Index: ui/keyboard/keyboard_switches.cc
diff --git a/ui/keyboard/keyboard_switches.cc b/ui/keyboard/keyboard_switches.cc
index d1ea4e2cbba4b7a9e2f187a17db7664230b35b3a..597d2d271d644e98731b6405bbb3c5f354fb5870 100644
--- a/ui/keyboard/keyboard_switches.cc
+++ b/ui/keyboard/keyboard_switches.cc
@@ -8,6 +8,7 @@ namespace keyboard {
 namespace switches {
 
 const char kEnableVirtualKeyboard[] = "enable-virtual-keyboard";
+const char kKeyboardUsabilityTest[] = "keyboard-usability-test";
 
 }  // namespace switches
 }  // namespace keyboard