Add user-agent flag to headless

headless/app/headless_shell_switches.cc

Index: headless/app/headless_shell_switches.cc
diff --git a/headless/app/headless_shell_switches.cc b/headless/app/headless_shell_switches.cc
index f20a60657f77c388713a5828ba428fe600e2d35a..ec47464f63a7b98042c99aecf5ac34486bea3c50 100644
--- a/headless/app/headless_shell_switches.cc
+++ b/headless/app/headless_shell_switches.cc
@@ -60,5 +60,8 @@ const char kVirtualTimeBudget[] = "virtual-time-budget";
 // Sets the initial window size. Provided as string in the format "800,600".
 const char kWindowSize[] = "window-size";
 
+// A string used to override the default user agent with a custom one.
+const char kUserAgent[] = "user-agent";
+
 }  // namespace switches
 }  // namespace headless