sync: Add pre-commit update avoidance mode + flag

sync/sessions/sync_session_context.h

Index: sync/sessions/sync_session_context.h
diff --git a/sync/sessions/sync_session_context.h b/sync/sessions/sync_session_context.h
index b694162c15637e046d1997f51d0d8bcde6d42cb2..76170958eccec5441e4a937cfaaafadffec8765a 100644
--- a/sync/sessions/sync_session_context.h
+++ b/sync/sessions/sync_session_context.h
@@ -55,6 +55,7 @@ class SYNC_EXPORT_PRIVATE 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);
 
   ~SyncSessionContext();
@@ -129,6 +130,15 @@ class SYNC_EXPORT_PRIVATE SyncSessionContext {
     return invalidator_client_id_;
   }
 
+  bool ShouldFetchUpdatesBeforeCommit() const {
+    return !(server_enabled_pre_commit_update_avoidance_ ||
+             client_enabled_pre_commit_update_avoidance_);
+  }
+
+  void set_server_enabled_pre_commit_update_avoidance(bool value) {
+    server_enabled_pre_commit_update_avoidance_ = value;
+  }
+
  private:
   // Rather than force clients to set and null-out various context members, we
   // extend our encapsulation boundary to scoped helpers that take care of this
@@ -181,6 +191,15 @@ class SYNC_EXPORT_PRIVATE SyncSessionContext {
   // prevent us from receiving notifications of changes we make ourselves.
   const std::string invalidator_client_id_;
 
+  // Flag to enable or disable the no pre-commit GetUpdates experiment.  When
+  // this flag is set to false, the syncer has the option of not performing at
+  // GetUpdates request when there is nothing to fetch.
+  bool server_enabled_pre_commit_update_avoidance_;
+
+  // If true, indicates that we've been passed a command-line flag to force
+  // enable the pre-commit update avoidance experiment described above.
+  const bool client_enabled_pre_commit_update_avoidance_;
+
   DISALLOW_COPY_AND_ASSIGN(SyncSessionContext);
 };