[Sync] Stop accessing BrowserContextKeyedServiceFactory on non-UI thread.

components/sync/driver/sync_client.h

Index: components/sync/driver/sync_client.h
diff --git a/components/sync/driver/sync_client.h b/components/sync/driver/sync_client.h
index ddf64d83d97675a279f0ec22c5b5b687b7e78974..c0030b4cc2a9d18614cc90ee9f38b55a019d4209 100644
--- a/components/sync/driver/sync_client.h
+++ b/components/sync/driver/sync_client.h
@@ -59,6 +59,8 @@ class SyncableService;
 // to handle these scenarios gracefully.
 class SyncClient {
  public:
+  using ServiceProvider = base::Callback<base::WeakPtr<SyncableService>()>;
+
   SyncClient();
   virtual ~SyncClient();
 
@@ -100,11 +102,10 @@ class SyncClient {
   virtual scoped_refptr<ExtensionsActivity> GetExtensionsActivity() = 0;
   virtual sync_sessions::SyncSessionsClient* GetSyncSessionsClient() = 0;
 
-  // Returns a weak pointer to the syncable service specified by |type|.
-  // Weak pointer may be unset if service is already destroyed.
-  // Note: Should only be dereferenced from the model type thread.
-  virtual base::WeakPtr<SyncableService> GetSyncableServiceForType(
-      ModelType type) = 0;
+  // Returns a callback to retrieve a syncable service specified by |type|.
+  // Both the provider and the resulting weak pointer will only be accessed on
+  // the model thread.
+  virtual ServiceProvider GetSyncableServiceForType(ModelType type) = 0;
 
   // Returns a weak pointer to the ModelTypeSyncBridge specified by |type|. Weak
   // pointer may be unset if service is already destroyed.