Add a switch for tablet power button behavior

ash/system/chromeos/power/tablet_power_button_controller_unittest.cc

Index: ash/system/chromeos/power/tablet_power_button_controller_unittest.cc
diff --git a/ash/system/chromeos/power/tablet_power_button_controller_unittest.cc b/ash/system/chromeos/power/tablet_power_button_controller_unittest.cc
index dc675abb2e1bfb494c87e581a1679e14edc2ce57..1ab79a29c5bcca724dc5e238c62e22a470288140 100644
--- a/ash/system/chromeos/power/tablet_power_button_controller_unittest.cc
+++ b/ash/system/chromeos/power/tablet_power_button_controller_unittest.cc
@@ -50,6 +50,8 @@ class TabletPowerButtonControllerTest : public AshTestBase {
     dbus_setter->SetPowerManagerClient(base::WrapUnique(power_manager_client_));
     base::CommandLine::ForCurrentProcess()->AppendSwitch(
         switches::kAshEnableTouchViewTesting);
+    base::CommandLine::ForCurrentProcess()->AppendSwitch(
+        switches::kAshEnableTabletPowerButton);
     AshTestBase::SetUp();
 
     lock_state_controller_ = Shell::GetInstance()->lock_state_controller();