Use Chromium BUILD to approximate Blimp protocol version, and check it.

blimp/common/version_info.h

Index: blimp/common/version_info.h
diff --git a/blimp/common/version_info.h b/blimp/common/version_info.h
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index 0000000000000000000000000000000000000000..5a6c20e05cc8ba40b1dafaba8db43746aaa0c922
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+++ b/blimp/common/version_info.h
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+// 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 BLIMP_COMMON_VERSION_INFO_H_
+#define BLIMP_COMMON_VERSION_INFO_H_
+
+#include <string>
+
+#include "blimp/common/blimp_common_export.h"
+
+namespace blimp {
+
+// Engine version to request during assignment.
+// TODO(wez): Merge this into GetProtocolVersion().
+std::string GetAssignmentRequestVersion();
+
+// Latest version of the protocol supported by this build.
+int BLIMP_COMMON_EXPORT GetProtocolVersion();
+
+}  // namespace blimp
+
+#endif  // BLIMP_COMMON_VERSION_INFO_H_