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

blimp/engine/app/blimp_system_url_request_context_getter.cc

Index: blimp/engine/app/blimp_system_url_request_context_getter.cc
diff --git a/blimp/engine/app/blimp_system_url_request_context_getter.cc b/blimp/engine/app/blimp_system_url_request_context_getter.cc
index 523a18b09aeee97b63f0acdee96973ba5c2e3743..c4c5bde3cd361056e6a783edb241883ca14463b2 100644
--- a/blimp/engine/app/blimp_system_url_request_context_getter.cc
+++ b/blimp/engine/app/blimp_system_url_request_context_getter.cc
@@ -9,6 +9,7 @@
 
 #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"
@@ -18,7 +19,6 @@ namespace blimp {
 namespace engine {
 
 BlimpSystemURLRequestContextGetter::BlimpSystemURLRequestContextGetter() {
-  // Must first be created on the UI thread.
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 }
 
@@ -30,9 +30,11 @@ BlimpSystemURLRequestContextGetter::GetURLRequestContext() {
   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();