Revert "Fix regression that dev-shortcut to enable tablet mode doesn't show overview button"

This reverts commit 561b1956.

Reason for revert: This solution is broken, will land a proper fix soon.

Original change's description:
> Fix regression that dev-shortcut to enable tablet mode doesn't show overview button
> 
> This was a rgression caused by a previous CL of mine https://crrev.com/c/1865543.
> Using the developer keyboard shortcut to enable tablet mode should
> force the overview tray button visible even though the internal
> input events are not being blocked.
> 
> BUG=1016594, 925087
> TEST=Manually, added a new test.
> 
> Change-Id: Ic9046c07dcce9de03c5fc5acd10a2077d599c711
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872468
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#708222}

TBR=xiyuan@chromium.org,afakhry@chromium.org

Change-Id: I98c49ad99d804242adaa7172de9e74337bdecda1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1016594, 925087
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874199Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708264}

ash/system/overview/overview_button_tray_unittest.cc

@@ -425,17 +425,4 @@ TEST_F(OverviewButtonTrayTest, LeaveTabletModeBecauseExternalMouse) {
   EXPECT_TRUE(GetTray()->GetVisible());
 }
 
-// Using the developers keyboard shortcut to enable tablet mode should force the
-// overview tray button visible, even though the events are not blocked.
-TEST_F(OverviewButtonTrayTest, ForDevTabletModeForcesTheButtonShown) {
-  Shell::Get()->tablet_mode_controller()->SetEnabledForDev(true);
-  EXPECT_TRUE(TabletModeControllerTestApi().IsTabletModeStarted());
-  EXPECT_FALSE(TabletModeControllerTestApi().AreEventsBlocked());
-  EXPECT_TRUE(GetTray()->GetVisible());
-
-  Shell::Get()->tablet_mode_controller()->SetEnabledForDev(false);
-  EXPECT_FALSE(TabletModeControllerTestApi().IsTabletModeStarted());
-  EXPECT_TRUE(GetTray()->GetVisible());
-}
-
 }  // namespace ash

ash/wm/tablet_mode/tablet_mode_controller.cc

@@ -184,9 +184,9 @@ constexpr TabletModeController::TabletModeBehavior kOnBySensor{
     /*force_physical_tablet_state=*/false,
 };
 
-// Defines the behavior that sticks to tablet mode. Used to implement the
-// --force-tablet-mode=touch_view flag.
-constexpr TabletModeController::TabletModeBehavior kLockInTabletMode{
+// Defines the behavior that sticks to the current mode. Used to
+// implement --force-tablet-mode flag.
+constexpr TabletModeController::TabletModeBehavior kLockInCurrentMode{
     /*use_sensor=*/false,
     /*observe_display_events=*/false,
     /*observe_external_pointer_device_events=*/false,
@@ -195,17 +195,6 @@ constexpr TabletModeController::TabletModeBehavior kLockInTabletMode{
     /*force_physical_tablet_state=*/false,
 };
 
-// Defines the behavior that sticks to tablet mode. Used to implement the
-// --force-tablet-mode=clamshell flag.
-constexpr TabletModeController::TabletModeBehavior kLockInClamshellMode{
-    /*use_sensor=*/false,
-    /*observe_display_events=*/false,
-    /*observe_external_pointer_device_events=*/false,
-    /*block_internal_input_device=*/false,
-    /*always_show_overview_button=*/false,
-    /*force_physical_tablet_state=*/false,
-};
-
 // Defines the behavior used for testing. It prevents the device from
 // switching the mode due to sensor events during the test.
 constexpr TabletModeController::TabletModeBehavior kOnForTest{
@@ -445,12 +434,8 @@ void TabletModeController::OnShellInitialized() {
   forced_ui_mode_ = GetUiMode();
   switch (forced_ui_mode_) {
     case UiMode::kTabletMode:
-      tablet_mode_behavior_ = kLockInTabletMode;
-      UpdateUiTabletState();
-      break;
-
     case UiMode::kClamshell:
-      tablet_mode_behavior_ = kLockInClamshellMode;
+      tablet_mode_behavior_ = kLockInCurrentMode;
       UpdateUiTabletState();
       break;
 
@@ -921,17 +906,8 @@ void TabletModeController::UpdateInternalInputDevicesEventBlocker() {
       tablet_mode_behavior_.block_internal_input_device &&
       (InTabletMode() || is_in_tablet_physical_state_);
 
-  if (should_block_internal_events == AreInternalInputDeviceEventsBlocked()) {
-    if (tablet_mode_behavior_.always_show_overview_button) {
-      // The overview button visibility gets updated based on the
-      // OnTabletModeEventsBlockingChanged() notification. Broadcast this event
-      // if we need to force it visible.
-      for (auto& observer : tablet_mode_observers_)
-        observer.OnTabletModeEventsBlockingChanged();
-    }
-
+  if (should_block_internal_events == AreInternalInputDeviceEventsBlocked())
     return;
-  }
 
   event_blocker_->UpdateInternalInputDevices(should_block_internal_events);
   for (auto& observer : tablet_mode_observers_)