chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/mus/test/wm_test_helper.cc

Index: ash/mus/test/wm_test_helper.cc
diff --git a/ash/mus/test/wm_test_helper.cc b/ash/mus/test/wm_test_helper.cc
index dc94727b60c313fa73f5433223b68c31db23957f..d3839cfeacda805ff7e9b3ae347b0c3f98cf614b 100644
--- a/ash/mus/test/wm_test_helper.cc
+++ b/ash/mus/test/wm_test_helper.cc
@@ -84,6 +84,9 @@ WmTestHelper::~WmTestHelper() {
 }
 
 void WmTestHelper::Init() {
+  // MaterialDesignController may have already been initialized. To cover that
+  // case explicitly uninitialize before initializing.
+  ui::test::MaterialDesignControllerTestAPI::Uninitialize();
   ui::MaterialDesignController::Initialize();
 
   views_delegate_ = base::MakeUnique<views::TestViewsDelegate>();