Add blimp to root GN check_targets

blimp/engine/common/blimp_user_agent.h

Index: blimp/engine/common/blimp_user_agent.h
diff --git a/blimp/engine/common/blimp_user_agent.h b/blimp/engine/common/blimp_user_agent.h
new file mode 100644
index 0000000000000000000000000000000000000000..e64af88712fcee7499ad3871ad7b4891f3dba9eb
--- /dev/null
+++ b/blimp/engine/common/blimp_user_agent.h
@@ -0,0 +1,21 @@
+// 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.
+
+#ifndef BLIMP_ENGINE_COMMON_BLIMP_USER_AGENT_H_
+#define BLIMP_ENGINE_COMMON_BLIMP_USER_AGENT_H_
+
+#include <string>
+
+#include "content/public/common/content_client.h"
+
+namespace blimp {
+namespace engine {
+
+std::string GetBlimpEngineUserAgent();
+void SetClientOSInfo(std::string client_os_info);
+
+}  // namespace engine
+}  // namespace blimp
+
+#endif  // BLIMP_ENGINE_COMMON_BLIMP_USER_AGENT_H_