Makes ScreenDimmer deal with the shell going away

Chrome may end up using ScreenDimmer in such a way that the ScreenDimmer is destroyed after the shell. This change makes ~ScreenDimmer deal with the shell having been destroyed so that it doesn't crash.

BUG=648723
TEST=covered by test
R=jamescook@chromium.org

Review-Url: https://codereview.chromium.org/2359633003
Cr-Commit-Position: refs/heads/master@{#419909}

ash/common/wm/screen_dimmer.cc

@@ -32,7 +32,9 @@ ScreenDimmer::ScreenDimmer(Container container)
 }
 
 ScreenDimmer::~ScreenDimmer() {
-  WmShell::Get()->RemoveShellObserver(this);
+  // Usage in chrome results in ScreenDimmer outliving the shell.
+  if (WmShell::HasInstance())
+    WmShell::Get()->RemoveShellObserver(this);
 }
 
 void ScreenDimmer::SetDimming(bool should_dim) {

ash/wm/screen_dimmer_unittest.cc

@@ -120,5 +120,27 @@ TEST_F(ScreenDimmerTest, DimAtBottom) {
   EXPECT_EQ(*dim_iter, *root_window->children().begin());
 }
 
+// See description above TEST_F for details.
+class ScreenDimmerShellDestructionTest : public AshTestBase {
+ public:
+  ScreenDimmerShellDestructionTest() {}
+  ~ScreenDimmerShellDestructionTest() override {}
+
+  void TearDown() override {
+    ScreenDimmer screen_dimmer(ScreenDimmer::Container::ROOT);
+    AshTestBase::TearDown();
+    // ScreenDimmer is destroyed *after* the shell.
+  }
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ScreenDimmerShellDestructionTest);
+};
+
+// This test verifies ScreenDimmer can be destroyed after the shell. The
+// interesting part of this test is in TearDown(), which creates a ScreenDimmer
+// that is deleted after WmShell.
+TEST_F(ScreenDimmerShellDestructionTest, DontCrashIfScreenDimmerOutlivesShell) {
+}
+
 }  // namespace test
 }  // namespace ash