chromeos: makes more tests run in both mash and ash

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 546c86121c65e842358446a28cfd5456ece5b887..8a2df301343d4768b12ed7e479d5dbeee0cf6829 100644
--- a/ash/system/chromeos/power/tablet_power_button_controller_unittest.cc
+++ b/ash/system/chromeos/power/tablet_power_button_controller_unittest.cc
@@ -75,8 +75,11 @@ class TabletPowerButtonControllerTest : public AshTestBase {
 
   void TearDown() override {
     generator_ = nullptr;
+    const bool is_mash = WmShell::Get()->IsRunningInMash();
     AshTestBase::TearDown();
-    chromeos::DBusThreadManager::Shutdown();
+    // Mash shuts down dbus after each test.
+    if (!is_mash)
+      chromeos::DBusThreadManager::Shutdown();
   }
 
  protected: