Fix crash on referencing deleted SignedInDevicesManager observer

Each profile creates new instance of SignedInDevicesManager so, for example,
opening incognito window and then closing it, destroyes an instance of
SignedInDevicesManager created for this profile. The code has forgot to remove
itself from the list of observers in EventRouter.
That caused crash when that event was triggered later in a normal profile as
EventRouter would reference stale pointer.

In tests observer is not added and profile_ is null so null-checking it.

R=rdevlin.cronin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/980663002

Cr-Commit-Position: refs/heads/master@{#319755}

chrome/browser/extensions/api/signed_in_devices/signed_in_devices_manager.cc

@@ -104,7 +104,7 @@ SignedInDevicesManager::SignedInDevicesManager()
 SignedInDevicesManager::SignedInDevicesManager(content::BrowserContext* context)
     : profile_(Profile::FromBrowserContext(context)),
       extension_registry_observer_(this) {
-  extensions::EventRouter* router = extensions::EventRouter::Get(profile_);
+  EventRouter* router = EventRouter::Get(profile_);
   if (router) {
     router->RegisterObserver(
         this, api::signed_in_devices::OnDeviceInfoChange::kEventName);
@@ -115,7 +115,13 @@ SignedInDevicesManager::SignedInDevicesManager(content::BrowserContext* context)
   extension_registry_observer_.Add(ExtensionRegistry::Get(profile_));
 }
 
-SignedInDevicesManager::~SignedInDevicesManager() {}
+SignedInDevicesManager::~SignedInDevicesManager() {
+  if (profile_) {
+    EventRouter* router = EventRouter::Get(profile_);
+    if (router)
+      router->UnregisterObserver(this);
+  }
+}
 
 void SignedInDevicesManager::OnListenerAdded(
     const EventListenerInfo& details) {