[Sync] Add sync_export.h (Step 1 for componentizing sync)

sync/base/sync_export.h

Index: sync/base/sync_export.h
diff --git a/sync/base/sync_export.h b/sync/base/sync_export.h
new file mode 100644
index 0000000000000000000000000000000000000000..a38e8b23a365e01b1bee96c4985e07fa6f28a740
--- /dev/null
+++ b/sync/base/sync_export.h
@@ -0,0 +1,41 @@
+// 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.
+
+#ifndef SYNC_SYNC_EXPORT_H_
+#define SYNC_SYNC_EXPORT_H_
+
+// TODO(akalin): Remove '0 && ' when sync becomes a component.
+#if 0 && defined(COMPONENT_BUILD)
+#if defined(WIN32)
+
+#if defined(SYNC_IMPLEMENTATION)
+#define SYNC_EXPORT __declspec(dllexport)
+#define SYNC_EXPORT_PRIVATE __declspec(dllexport)
+#elif defined(SYNC_TEST)
+#define SYNC_EXPORT __declspec(dllimport)
+#define SYNC_EXPORT_PRIVATE __declspec(dllimport)
+#else
+#define SYNC_EXPORT __declspec(dllimport)
+#define SYNC_EXPORT_PRIVATE
+#endif  // defined(SYNC_IMPLEMENTATION)
+
+#else  // defined(WIN32)
+#if defined(SYNC_IMPLEMENTATION)
+#define SYNC_EXPORT __attribute__((visibility("default")))
+#define SYNC_EXPORT_PRIVATE __attribute__((visibility("default")))
+#elif defined(SYNC_TEST)
+#define SYNC_EXPORT
+#define SYNC_EXPORT_PRIVATE __attribute__((visibility("default")))
+#else
+#define SYNC_EXPORT
+#define SYNC_EXPORT_PRIVATE
+#endif  // defined(SYNC_IMPLEMENTATION)
+#endif
+
+#else  // defined(COMPONENT_BUILD)
+#define SYNC_EXPORT
+#define SYNC_EXPORT_PRIVATE
+#endif
+
+#endif  // SYNC_SYNC_EXPORT_H_