[SyncFS] Initialize SyncWorker when sync is enabled.

chrome/browser/sync_file_system/drive_backend/sync_engine.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sync_file_system/drive_backend/sync_engine.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/metrics/histogram.h"
@@ skipping 558 matching lines @@
       callback_tracker_.Register(abort_closure, callback);
 
   PostTaskAndReplyWithResult(worker_task_runner_.get(),
                              FROM_HERE,
                              base::Bind(&SyncWorkerInterface::DumpDatabase,
                                         base::Unretained(sync_worker_.get())),
                              tracked_callback);
 }
 
 void SyncEngine::SetSyncEnabled(bool sync_enabled) {
+  if (sync_enabled_ == sync_enabled)
+    return;
   sync_enabled_ = sync_enabled;
 
+  if (sync_enabled_) {
+    if (!sync_worker_)
+      Initialize();
+
+    // Have no login credential.
+    if (!sync_worker_)
+      return;
+
+    worker_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(&SyncWorkerInterface::SetSyncEnabled,
+                   base::Unretained(sync_worker_.get()),
+                   sync_enabled_));
+    return;
+  }
+
   if (!sync_worker_)
     return;
 
+  // TODO(tzik): Consider removing SyncWorkerInterface::SetSyncEnabled and
+  // let SyncEngine handle the flag.
   worker_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&SyncWorkerInterface::SetSyncEnabled,
                  base::Unretained(sync_worker_.get()),
-                 sync_enabled));
+                 sync_enabled_));
+  Reset();
 }
 
 void SyncEngine::PromoteDemotedChanges(const base::Closure& callback) {
   if (!sync_worker_) {
     callback.Run();
     return;
   }
 
   base::Closure relayed_callback = RelayCallbackToCurrentThread(
       FROM_HERE, callback_tracker_.Register(callback, callback));
@@ skipping 183 matching lines @@
 
 SyncStatusCallback SyncEngine::TrackCallback(
     const SyncStatusCallback& callback) {
   return callback_tracker_.Register(
       base::Bind(callback, SYNC_STATUS_ABORT),
       callback);
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system