add a LinuxSandbox::HasOpenDirectories() sanity check

content/common/sandbox_linux.h

Index: content/common/sandbox_linux.h
diff --git a/content/common/sandbox_linux.h b/content/common/sandbox_linux.h
index 412c04ab20bdf1915974443370025d853579ed9b..6011374316f8ceddd85a34f05c6e993e042e217e 100644
--- a/content/common/sandbox_linux.h
+++ b/content/common/sandbox_linux.h
@@ -57,6 +57,9 @@ class LinuxSandbox {
   // Returns true if the current process is single-threaded or if the number
   // of threads cannot be determined.
   bool IsSingleThreaded() const;
+  // Returns true if it can be determined that the current process has open
+  // directories (besides /proc).

[jln (very slow on Chromium), 2013/10/22 01:10:50]

Let's just say "that are not managed by the LinuxSandbox class". We could even
add: "This would be a vulnerability as it would allow to bypass the setuid
sandbox".

Let's also make it private for now.

[Mostyn Bramley-Moore, 2013/10/23 23:15:19]

Done in patchset 2.

+  bool HasOpenDirectories() const;
   // Did we start Seccomp BPF?
   bool seccomp_bpf_started() const;