chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/mus/window_manager_unittest.cc

Index: ash/mus/window_manager_unittest.cc
diff --git a/ash/mus/window_manager_unittest.cc b/ash/mus/window_manager_unittest.cc
index 4c2c0cd0e88246778ebfd775b94812bdbd1a1108..60e9862b46d44f2e237d7cf3f1c27f62f6e44c48 100644
--- a/ash/mus/window_manager_unittest.cc
+++ b/ash/mus/window_manager_unittest.cc
@@ -66,7 +66,28 @@ class WindowManagerTest : public service_manager::test::ServiceTest {
   WindowManagerTest() : service_manager::test::ServiceTest("mash_unittests") {}
   ~WindowManagerTest() override {}
 
+  // service_manager::test::ServiceTest:
+  void SetUp() override {
+    service_manager::test::ServiceTest::SetUp();
+
+    // This test triggers a path that clobbers the context_factory set on Env.

[msw, 2017/02/19 21:49:20]

Can you elaborate on the exact mechanism that clobbers the context factory? (Is
it ash::Shell via AshTestHelper?) It's not obvious here. It would be far nicer
if that mechanism were able to restore the original factory itself. (Could
ash::Shell or AshTestHelper do that? call TerminateContextFactoryForTests() or
similar?)

[sky, 2017/02/21 17:06:50]

Done.
This code doesn't use any of the AshTest* classes, but it inherits the Env
created in MashTestSuite. TerminateContextFactoryForTests() wouldn't work well
in this case either as it doesn't reset env.

+    // As all tests share the same Env (see mash_test_suite) we need to restore
+    // the context_factory when done.
+    aura::Env* env = aura::Env::GetInstance();
+    initial_context_factory_ = env->context_factory();
+    initial_context_factory_private_ = env->context_factory_private();
+  }
+
+  void TearDown() override {
+    aura::Env* env = aura::Env::GetInstance();
+    env->set_context_factory(initial_context_factory_);
+    env->set_context_factory_private(initial_context_factory_private_);
+  }
+
  private:
+  ui::ContextFactory* initial_context_factory_ = nullptr;
+  ui::ContextFactoryPrivate* initial_context_factory_private_ = nullptr;
+
   DISALLOW_COPY_AND_ASSIGN(WindowManagerTest);
 };