Add user agent to System Context Getter as suggested by metrics team.

blimp/engine/app/blimp_system_url_request_context_getter.cc

 // 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 "blimp/engine/app/blimp_system_url_request_context_getter.h"
 
 #include <utility>
 #include <vector>
 
 #include "base/logging.h"
 #include "base/single_thread_task_runner.h"
+#include "blimp/engine/common/blimp_user_agent.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/proxy/proxy_service.h"
 #include "net/url_request/url_request_context.h"
 #include "net/url_request/url_request_context_builder.h"
 
 namespace blimp {
 namespace engine {
 
 BlimpSystemURLRequestContextGetter::BlimpSystemURLRequestContextGetter() {
-  // Must first be created on the UI thread.
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 }
 
 BlimpSystemURLRequestContextGetter::~BlimpSystemURLRequestContextGetter() {}
 
 net::URLRequestContext*
 BlimpSystemURLRequestContextGetter::GetURLRequestContext() {
   DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
   if (!url_request_context_) {
     // Use default values
     net::URLRequestContextBuilder builder;
+
     // TODO(jessicag): See if proxy_service setup should be harmonized with
     // user request context getter. http://crbug/609981
     builder.set_proxy_service(net::ProxyService::CreateDirect());
+    builder.set_user_agent(GetBlimpEngineUserAgent());
     url_request_context_ = builder.Build();
   }
   return url_request_context_.get();
 }
 
 scoped_refptr<base::SingleThreadTaskRunner>
 BlimpSystemURLRequestContextGetter::GetNetworkTaskRunner() const {
   return content::BrowserThread::GetMessageLoopProxyForThread(
       content::BrowserThread::IO);
 }
 
 }  // namespace engine
 }  // namespace blimp