Linux Sandbox: add Credentials::SupportsNewUserNS()

sandbox/linux/services/credentials.h

Index: sandbox/linux/services/credentials.h
diff --git a/sandbox/linux/services/credentials.h b/sandbox/linux/services/credentials.h
index c23db930df270ea39d95845fadda268f94b01cd5..48cf259a2f9eca1b78c5a8bc66caedaa847bed54 100644
--- a/sandbox/linux/services/credentials.h
+++ b/sandbox/linux/services/credentials.h
@@ -51,6 +51,13 @@ class Credentials {
   // debugging and tests.
   scoped_ptr<std::string> GetCurrentCapString() const;
 
+  // Returns whether the kernel supports CLONE_NEWUSER and whether it would be
+  // possible to immediately move to a new user namespace. There is no point
+  // in using this method right before calling MoveToNewUserNS(), simply call
+  // MoveToNewUserNS() immediately. This method is only useful to test kernel
+  // support ahead of time.
+  static bool SupportsNewUserNS();
+
   // Move the current process to a new "user namespace" as supported by Linux
   // 3.8+ (CLONE_NEWUSER).
   // The uid map will be set-up so that the perceived uid and gid will not