Reland of Mojo: Eliminate duplicate C API symbols

mojo/public/c/system/set_thunks_for_app.cc

Index: mojo/public/c/system/set_thunks_for_app.cc
diff --git a/mojo/public/c/system/set_thunks_for_app.cc b/mojo/public/c/system/set_thunks_for_app.cc
new file mode 100644
index 0000000000000000000000000000000000000000..335cc02b79477bb45f28208414feb74d2f10471c
--- /dev/null
+++ b/mojo/public/c/system/set_thunks_for_app.cc
@@ -0,0 +1,20 @@
+// Copyright 2016 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.
+
+#include "mojo/public/c/system/thunks.h"
+
+extern "C" {
+
+#if defined(WIN32)
+#define THUNKS_EXPORT __declspec(dllexport)
+#else
+#define THUNKS_EXPORT __attribute__((visibility("default")))
+#endif
+
+THUNKS_EXPORT size_t MojoSetSystemThunks(
+    const MojoSystemThunks* system_thunks) {
+  return MojoEmbedderSetSystemThunks(system_thunks);
+}
+
+}  // extern "C"