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

Currently the user agent is set to empty string which results in the
upload being ignored by the metrics service.

Setting this value will allow for processing and display of metrics
for Blimp.

BUG=592757

Review-Url: https://codereview.chromium.org/2041933002
Cr-Commit-Position: refs/heads/master@{#398155}

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();