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 85083ae0700872d152bf9b79d046c131da4f3342..c06bf1fa44fbbfe6512a14b4e13e51ab92aeabca 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -41,6 +41,9 @@ const char kAuditAllHandles[]               = "enable-handle-auditing-all";
 // is terminated.
 const char kAuditHandles[]                  = "enable-handle-auditing";
 
+// Prefer the namespace sandbox over the setuid sandbox when possible.
+const char kEnableNamespaceSandbox[]        = "enable-namespace-sandbox";
+
 // Choose which logging channels in blink platform to activate.  See
 // Logging.cpp in blink's Source/platform for a list of available channels.
 const char kBlinkPlatformLogChannels[]      = "blink-platform-log-channels";