tablet: Replace redundant if statement with DCHECK

Concerning TabletModeController, the if statement at the beginning of
SetTabletModeEnabledInternal checks the same condition as the if
statement in UpdateUiTabletState. It does not need to be checked twice.

Bug: None
Change-Id: I2e69c1f34c476f819ed42aa5dd66f46f1022d8e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090206
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747936}

ash/wm/tablet_mode/tablet_mode_controller.cc

@@ -293,7 +293,7 @@ class TabletModeController::DestroyObserver : public aura::WindowObserver {
 constexpr char TabletModeController::kLidAngleHistogramName[];
 
 ////////////////////////////////////////////////////////////////////////////////
-// TabletModeContrller, public:
+// TabletModeController, public:
 
 // static
 void TabletModeController::SetUseScreenshotForTest(bool use_screenshot) {
@@ -683,11 +683,10 @@ bool TabletModeController::ShouldShowOverviewButton() const {
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-// TabletModeContrller, private:
+// TabletModeController, private:
 
 void TabletModeController::SetTabletModeEnabledInternal(bool should_enable) {
-  if (InTabletMode() == should_enable)
-    return;
+  DCHECK_NE(InTabletMode(), should_enable);
 
   // Hide the context menu on entering tablet mode to prevent users from
   // accessing forbidden options. Hide the context menu on exiting tablet mode