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/+/1872468Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708222}

ash/system/overview/overview_button_tray_unittest.cc

@@ -425,4 +425,17 @@ 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 the current mode. Used to
-// implement --force-tablet-mode flag.
-constexpr TabletModeController::TabletModeBehavior kLockInCurrentMode{
+// Defines the behavior that sticks to tablet mode. Used to implement the
+// --force-tablet-mode=touch_view flag.
+constexpr TabletModeController::TabletModeBehavior kLockInTabletMode{
     /*use_sensor=*/false,
     /*observe_display_events=*/false,
     /*observe_external_pointer_device_events=*/false,
@@ -195,6 +195,17 @@ constexpr TabletModeController::TabletModeBehavior kLockInCurrentMode{
     /*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{
@@ -434,8 +445,12 @@ 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_ = kLockInCurrentMode;
+      tablet_mode_behavior_ = kLockInClamshellMode;
       UpdateUiTabletState();
       break;
 
@@ -906,8 +921,17 @@ void TabletModeController::UpdateInternalInputDevicesEventBlocker() {
       tablet_mode_behavior_.block_internal_input_device &&
       (InTabletMode() || is_in_tablet_physical_state_);
 
-  if (should_block_internal_events == AreInternalInputDeviceEventsBlocked())
+  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();
+    }
+
     return;
+  }
 
   event_blocker_->UpdateInternalInputDevices(should_block_internal_events);
   for (auto& observer : tablet_mode_observers_)