Fix crash in ImmersiveFocusWatcherMus during shutdown.

Test: browser_tests
Bug: none
Change-Id: I2f4841af05d1ca2b0ea3ad686e8e1b322549b3d2
Reviewed-on: https://chromium-review.googlesource.com/1197903Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587844}

chrome/browser/ui/views/frame/immersive_focus_watcher_mus.cc

@@ -172,7 +172,9 @@ ImmersiveFocusWatcherMus::ImmersiveFocusWatcherMus(
 ImmersiveFocusWatcherMus::~ImmersiveFocusWatcherMus() {
   aura::client::GetTransientWindowClient()->RemoveObserver(this);
   GetWidget()->GetFocusManager()->RemoveFocusChangeListener(this);
-  ::wm::GetActivationClient(GetWidgetWindow())->RemoveObserver(this);
+  auto* activation_client = ::wm::GetActivationClient(GetWidgetWindow());
+  if (activation_client)
+    activation_client->RemoveObserver(this);
 }
 
 void ImmersiveFocusWatcherMus::UpdateFocusRevealedLock() {