Export all CWV* symbols from the ios/web_view dynamic library.

ios/web_view/public/cwv_export.h

+// Copyright 2017 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 IOS_WEB_VIEW_PUBLIC_CWV_EXPORT_H_
+#define IOS_WEB_VIEW_PUBLIC_CWV_EXPORT_H_
+
+// All public classes in //ios/web_view must be prefixed with CWV_EXPORT, so
+// that clients of //ios/web_view dynamic library can link them.
+//
+// This is because a dynamic library only exports symbols marked
+// __attribute__((visibility("default"))).
+//
+// Define a macro CWV_EXPORT instead of directly using __attribute__.
+// This makes it possible to export symbols only when *building* the dynamic
+// library (by checking CWV_IMPLEMENTATION), not when *using* the dynamic
+// library.
+#if defined(CWV_IMPLEMENTATION)
+#define CWV_EXPORT __attribute__((visibility("default")))
+#else
+#define CWV_EXPORT
+#endif
+
+#endif  // IOS_WEB_VIEW_PUBLIC_CWV_EXPORT_H_