chromeos: Enable mode-specific power button behavior.

Enable mode-specific power button behavior by default. In
short, tapping the power button turns the display off on
devices that are in tablet mode but is ignored on devices
that are in laptop mode.

Bug: 819276, 783164
Change-Id: Ic658e034366a32066940c647a73b8c61ecec2840
Reviewed-on: https://chromium-review.googlesource.com/1014486
Commit-Queue: Dan Erat <derat@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Reviewed-by: Qiang Xu <warx@google.com>
Cr-Commit-Position: refs/heads/master@{#551427}

ash/accelerators/accelerator_controller_unittest.cc

@@ -1012,6 +1012,7 @@ TEST_F(PreferredReservedAcceleratorsTest, AcceleratorsWithFullscreen) {
   ui::test::EventGenerator& generator = GetEventGenerator();
 
   // Power key (reserved) should always be handled.
+  Shell::Get()->power_button_controller()->OnTabletModeStarted();
   PowerButtonControllerTestApi test_api(
       Shell::Get()->power_button_controller());
   EXPECT_FALSE(test_api.PowerButtonMenuTimerIsRunning());
@@ -1061,6 +1062,7 @@ TEST_F(PreferredReservedAcceleratorsTest, AcceleratorsWithPinned) {
   ui::test::EventGenerator& generator = GetEventGenerator();
 
   // Power key (reserved) should always be handled.
+  Shell::Get()->power_button_controller()->OnTabletModeStarted();
   PowerButtonControllerTestApi test_api(
       Shell::Get()->power_button_controller());
   EXPECT_FALSE(test_api.PowerButtonMenuTimerIsRunning());

ash/lock_screen_action/lock_screen_note_display_state_handler_unittest.cc

@@ -196,6 +196,7 @@ class LockScreenNoteDisplayStateHandlerTest : public AshTestBase {
         chromeos::PowerManagerClient::SwitchStates{
             chromeos::PowerManagerClient::LidState::OPEN,
             chromeos::PowerManagerClient::TabletMode::ON});
+    Shell::Get()->power_button_controller()->OnTabletModeStarted();
   }
 
   base::SimpleTestTickClock tick_clock_;

ash/public/cpp/ash_features.cc

@@ -35,7 +35,7 @@ const base::Feature kLockScreenNotifications{"LockScreenNotifications",
                                              base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kModeSpecificPowerButton{"ModeSpecificPowerButton",
-                                             base::FEATURE_DISABLED_BY_DEFAULT};
+                                             base::FEATURE_ENABLED_BY_DEFAULT};
 
 bool IsDisplayMoveWindowAccelsEnabled() {
   return base::FeatureList::IsEnabled(kDisplayMoveWindowAccels);