sync: Add pre-commit update avoidance mode + flag

sync/sessions/sync_session_context.cc

Index: sync/sessions/sync_session_context.cc
diff --git a/sync/sessions/sync_session_context.cc b/sync/sessions/sync_session_context.cc
index 9043db3cbae1ab6555b34e0aee2a38ce64429fd3..c48c32b31ee9f8c21d32ee9d498e1cce54d48d2a 100644
--- a/sync/sessions/sync_session_context.cc
+++ b/sync/sessions/sync_session_context.cc
@@ -22,6 +22,7 @@ SyncSessionContext::SyncSessionContext(
     DebugInfoGetter* debug_info_getter,
     TrafficRecorder* traffic_recorder,
     bool keystore_encryption_enabled,
+    bool client_enabled_pre_commit_update_avoidance,
     const std::string& invalidator_client_id)
     : connection_manager_(connection_manager),
       directory_(directory),
@@ -31,7 +32,10 @@ SyncSessionContext::SyncSessionContext(
       debug_info_getter_(debug_info_getter),
       traffic_recorder_(traffic_recorder),
       keystore_encryption_enabled_(keystore_encryption_enabled),
-      invalidator_client_id_(invalidator_client_id) {
+      invalidator_client_id_(invalidator_client_id),
+      server_enabled_pre_commit_update_avoidance_(false),
+      client_enabled_pre_commit_update_avoidance_(
+          client_enabled_pre_commit_update_avoidance) {
   for (size_t i = 0u; i < workers.size(); ++i)
     workers_.push_back(workers[i]);