Makes ScreenDimmer deal with the shell going away

ash/common/wm/screen_dimmer.cc

Index: ash/common/wm/screen_dimmer.cc
diff --git a/ash/common/wm/screen_dimmer.cc b/ash/common/wm/screen_dimmer.cc
index b00ac9052f783197bc776c92630e9d1383a19883..00e936ae2d2ab10bccb6153bd5ef413b6538c69b 100644
--- a/ash/common/wm/screen_dimmer.cc
+++ b/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) {