app_list::StartPageService: Document and check nulls.

This adds null checks for the result of StartPageService::Get in a few
places where it was missing.

Review URL: https://codereview.chromium.org/970863003

Cr-Commit-Position: refs/heads/master@{#318985}

chrome/browser/ui/app_list/start_page_service.h

@@ -55,7 +55,7 @@ class StartPageService : public KeyedService,
  public:
   typedef std::vector<scoped_refptr<const extensions::Extension> >
       ExtensionList;
-  // Gets the instance for the given profile.
+  // Gets the instance for the given profile. May return nullptr.
   static StartPageService* Get(Profile* profile);
 
   void AddObserver(StartPageObserver* observer);

chrome/browser/ui/app_list/start_page_service_factory.h

@@ -16,7 +16,8 @@ class StartPageService;
 // Singleton factory to create StartPageService.
 class StartPageServiceFactory : public BrowserContextKeyedServiceFactory {
  public:
-  // Gets or creates the instance of StartPageService for |profile|.
+  // Gets or creates the instance of StartPageService for |profile|. May return
+  // nullptr.
   static StartPageService* GetForProfile(Profile* profile);
 
   // Gets the singleton instance of this factory.

chrome/browser/ui/ash/app_list/app_list_service_ash.cc

@@ -56,7 +56,10 @@ void AppListServiceAsh::ShowAndSwitchToState(
 void AppListServiceAsh::Init(Profile* initial_profile) {
   // Ensure the StartPageService is created here. This early initialization is
   // necessary to allow the WebContents to load before the app list is shown.
-  app_list::StartPageService::Get(initial_profile)->Init();
+  app_list::StartPageService* service =
+      app_list::StartPageService::Get(initial_profile);
+  if (service)
+    service->Init();
 }
 
 base::FilePath AppListServiceAsh::GetProfilePath(

chrome/browser/ui/webui/app_list/start_page_handler.cc

@@ -127,7 +127,7 @@ void StartPageHandler::OnHotwordEnabledChanged() {
     StartPageService* service = StartPageService::Get(profile);
     web_ui()->CallJavascriptFunction(
         "appList.startPage.setHotwordEnabled",
-        base::FundamentalValue(service->HotwordEnabled()));
+        base::FundamentalValue(service && service->HotwordEnabled()));
   }
 }
 #endif