[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_context.h

Index: content/browser/background_fetch/background_fetch_context.h
diff --git a/content/browser/background_fetch/background_fetch_context.h b/content/browser/background_fetch/background_fetch_context.h
index 37a1c94adeb9e9850aa531a85b704f5e8a8ef98c..73589e9370588279f647eee8de9d92848c21eb82 100644
--- a/content/browser/background_fetch/background_fetch_context.h
+++ b/content/browser/background_fetch/background_fetch_context.h
@@ -39,11 +39,11 @@ class StoragePartitionImpl;
 
 // The BackgroundFetchContext is the central moderator of ongoing background
 // fetch requests from the Mojo service and from other callers.
-// Background Fetch requests function similar to normal fetches except that
+// Background Fetch requests function similarly to normal fetches except that
 // they are persistent across Chromium or service worker shutdown.
 class CONTENT_EXPORT BackgroundFetchContext
     : public base::RefCountedThreadSafe<BackgroundFetchContext,
-                                        BrowserThread::DeleteOnUIThread> {
+                                        BrowserThread::DeleteOnIOThread> {
  public:
   // The BackgroundFetchContext will watch the ServiceWorkerContextWrapper so
   // that it can respond to service worker events such as unregister.
@@ -56,9 +56,6 @@ class CONTENT_EXPORT BackgroundFetchContext
   void InitializeOnIOThread(
       scoped_refptr<net::URLRequestContextGetter> request_context_getter);
 
-  // Shutdown must be called before deleting this. Call on the UI thread.
-  void Shutdown();
-
   // Starts a Background Fetch for the |registration_id|. The |requests| will be
   // asynchronously fetched. The |callback| will be invoked when the fetch has
   // been registered, or an error occurred that avoids it from doing so.
@@ -80,15 +77,12 @@ class CONTENT_EXPORT BackgroundFetchContext
 
  private:
   friend class base::DeleteHelper<BackgroundFetchContext>;
-  friend struct BrowserThread::DeleteOnThread<BrowserThread::UI>;
+  friend struct BrowserThread::DeleteOnThread<BrowserThread::IO>;
   friend class base::RefCountedThreadSafe<BackgroundFetchContext,
-                                          BrowserThread::DeleteOnUIThread>;
+                                          BrowserThread::DeleteOnIOThread>;
 
   ~BackgroundFetchContext();
 
-  // Shuts down the active Job Controllers on the IO thread.
-  void ShutdownOnIO();
-
   // Creates a new Job Controller for the given |registration_id| and |options|,
   // which will start fetching the files that are part of the registration.
   void CreateController(