Add user-agent flag to headless

headless/app/headless_shell.cc

Index: headless/app/headless_shell.cc
diff --git a/headless/app/headless_shell.cc b/headless/app/headless_shell.cc
index d6d6adeb758a5ef06ce2821d3973911297b22c74..feeb322cb7dec35de7533967e1fcb8e904a6b50c 100644
--- a/headless/app/headless_shell.cc
+++ b/headless/app/headless_shell.cc
@@ -24,6 +24,7 @@
 #include "net/base/io_buffer.h"
 #include "net/base/ip_address.h"
 #include "net/base/net_errors.h"
+#include "net/http/http_util.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace headless {
@@ -519,6 +520,12 @@ int HeadlessShellMain(int argc, const char** argv) {
         WebPreferences* preferences) { preferences->hide_scrollbars = true; }));
   }
 
+  if (command_line.HasSwitch(switches::kUserAgent)) {
+    std::string ua = command_line.GetSwitchValueASCII(switches::kUserAgent);
+    if (net::HttpUtil::IsValidHeaderValue(ua))
+      builder.SetUserAgent(ua);
+  }
+
   return HeadlessBrowserMain(
       builder.Build(),
       base::Bind(&HeadlessShell::OnStart, base::Unretained(&shell)));