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

blimp/client/session/assignment_source.cc

Index: blimp/client/session/assignment_source.cc
diff --git a/blimp/client/session/assignment_source.cc b/blimp/client/session/assignment_source.cc
index 2a08376b59ae504baf4eaa180cf7621432301570..c708fb62b22a047bfc84a2f3f9a331b4da79592a 100644
--- a/blimp/client/session/assignment_source.cc
+++ b/blimp/client/session/assignment_source.cc
@@ -19,7 +19,7 @@
 #include "base/values.h"
 #include "blimp/client/app/blimp_client_switches.h"
 #include "blimp/common/get_client_token.h"
-#include "blimp/common/protocol_version.h"
+#include "blimp/common/version_info.h"
 #include "components/safe_json/safe_json_parser.h"
 #include "net/base/ip_address.h"
 #include "net/base/ip_endpoint.h"
@@ -243,7 +243,8 @@ void AssignmentSource::QueryAssigner(const std::string& client_auth_token) {
 
   // Write the JSON for the request data.
   base::DictionaryValue dictionary;
-  dictionary.SetString(kProtocolVersionKey, blimp::kEngineVersion);
+  dictionary.SetString(kProtocolVersionKey,
+                       base::IntToString(GetProtocolVersion()));
   std::string json;
   base::JSONWriter::Write(dictionary, &json);
   url_fetcher_->SetUploadData("application/json", json);