Add check for file system access to the sandbox.

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 b89a6aa7cf6ef861c3180ad99ee3f0018999ec10..157c8e75e8f960f47b1bbe64604bd905ad2521eb 100644
--- a/sandbox/linux/services/credentials.h
+++ b/sandbox/linux/services/credentials.h
@@ -94,6 +94,9 @@ class SANDBOX_EXPORT Credentials {
   //   - DropAllCapabilities() must be called to prevent escapes.
   static bool DropFileSystemAccess(int proc_fd) WARN_UNUSED_RESULT;
 
+  // This function returns true if the process can still access the filesystem.
+  static bool HasFileSystemAccess();
+
   // Forks and drops capabilities in the child.
   static pid_t ForkAndDropCapabilitiesInChild();