Add CHROMECAST_EXPORT definition for OEM shlib replacement

Needed for exposing graphics API to shared library replacement.

BUG=

Review URL: https://codereview.chromium.org/1036503003

Cr-Commit-Position: refs/heads/master@{#322096}

chromecast/chromecast.gyp

@@ -26,6 +26,14 @@
     ],
   },
   'targets': [
+    # Public API target for OEM partners to replace shlibs.
+    {
+      'target_name': 'cast_public_api',
+      'type': '<(component)',
+      'sources': [
+        'public/chromecast_export.h',
+      ],
+    },
     # TODO(gunsch): Remove this fake target once it's either added or no
     # longer referenced from internal code.
     {'target_name': 'cast_media_audio', 'type': 'none'},

chromecast/public/chromecast_export.h

+// Copyright 2015 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 CHROMECAST_PUBLIC_CHROMECAST_EXPORT_H_
+#define CHROMECAST_PUBLIC_CHROMECAST_EXPORT_H_
+
+// Export attribute for classes that are exposed in shared libraries,
+// allowing OEM partners to replace with their own implementations.
+#define CHROMECAST_EXPORT __attribute__((visibility("default")))
+
+#endif  // CHROMECAST_PUBLIC_CHROMECAST_EXPORT_H_