sync: Add pre-commit update avoidance mode + flag

sync/engine/syncer.cc

 // Copyright (c) 2012 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 "sync/engine/syncer.h"
 
 #include "base/debug/trace_event.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
@@ skipping 46 matching lines @@
 void Syncer::RequestEarlyExit() {
   base::AutoLock lock(early_exit_requested_lock_);
   early_exit_requested_ = true;
 }
 
 bool Syncer::NormalSyncShare(ModelTypeSet request_types,
                              const NudgeTracker& nudge_tracker,
                              SyncSession* session) {
   HandleCycleBegin(session);
   VLOG(1) << "Downloading types " << ModelTypeSetToString(request_types);
-  if (!DownloadAndApplyUpdates(
-          session,
-          base::Bind(&NormalDownloadUpdates,
-                     session,
-                     kCreateMobileBookmarksFolder,
-                     request_types,
-                     base::ConstRef(nudge_tracker)))) {
-    return HandleCycleEnd(session);
+  if (nudge_tracker.IsGetUpdatesRequired() ||
+      session->context()->should_fetch_updates_before_commit()) {
+    if (!DownloadAndApplyUpdates(
+            session,
+            base::Bind(&NormalDownloadUpdates,
+                       session,
+                       kCreateMobileBookmarksFolder,
+                       request_types,
+                       base::ConstRef(nudge_tracker)))) {
+      return HandleCycleEnd(session);
+    }
   }
 
   VLOG(1) << "Committing from types " << ModelTypeSetToString(request_types);
   SyncerError commit_result = BuildAndPostCommits(request_types, this, session);
   session->mutable_status_controller()->set_commit_result(commit_result);
 
   return HandleCycleEnd(session);
 }
 
 bool Syncer::ConfigureSyncShare(ModelTypeSet request_types,
@@ skipping 64 matching lines @@
 bool Syncer::HandleCycleEnd(SyncSession* session) {
   if (!ExitRequested()) {
     session->SendEventNotification(SyncEngineEvent::SYNC_CYCLE_ENDED);
     return true;
   } else {
     return false;
   }
 }
 
 }  // namespace syncer