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

components/sync/driver/fake_sync_client.cc

Index: components/sync/driver/fake_sync_client.cc
diff --git a/components/sync/driver/fake_sync_client.cc b/components/sync/driver/fake_sync_client.cc
index d33b56ae5a80c3eb9677f6b28a088ea0083f8705..8346be67677572e316b8da62aaebb8c8f1eca040 100644
--- a/components/sync/driver/fake_sync_client.cc
+++ b/components/sync/driver/fake_sync_client.cc
@@ -6,24 +6,17 @@
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
-#include "base/memory/weak_ptr.h"
 #include "components/sync/base/extensions_activity.h"
 #include "components/sync/base/sync_prefs.h"
 #include "components/sync/driver/fake_sync_service.h"
 
 namespace syncer {
 
-using ServiceProvider = SyncClient::ServiceProvider;
-
 namespace {
 
 void DummyRegisterPlatformTypesCallback(SyncService* sync_service,
                                         ModelTypeSet,
                                         ModelTypeSet) {}
-
-base::WeakPtr<SyncableService> EmptyWeakPtr() {
-  return base::WeakPtr<SyncableService>();
-}
 
 }  // namespace
 
@@ -108,8 +101,9 @@
   return nullptr;
 }
 
-ServiceProvider FakeSyncClient::GetSyncableServiceForType(ModelType type) {
-  return base::Bind(&EmptyWeakPtr);
+base::WeakPtr<SyncableService> FakeSyncClient::GetSyncableServiceForType(
+    ModelType type) {
+  return base::WeakPtr<SyncableService>();
 }
 
 base::WeakPtr<ModelTypeSyncBridge> FakeSyncClient::GetSyncBridgeForModelType(