Allow using the namespace sandbox in zygote host.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index cac4cd92057bf275732271f9368ce84744861161..2b9cf85db8e1708b73fef3835d5b904cc2958440 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -357,6 +357,9 @@ const char kEnableLogging[]                 = "enable-logging";
 // Enables the memory benchmarking extension
 const char kEnableMemoryBenchmarking[]      = "enable-memory-benchmarking";
 
+// Prefer the namespace sandbox over the setuid sandbox when possible.
+const char kEnableNamespaceSandbox[]        = "enable-namespace-sandbox";
+
 // Enables the network information API.
 const char kEnableNetworkInformation[]      = "enable-network-information";