Add check for file system access to the sandbox.

sandbox/linux/services/credentials.cc

Index: sandbox/linux/services/credentials.cc
diff --git a/sandbox/linux/services/credentials.cc b/sandbox/linux/services/credentials.cc
index 0c617d4b2f19454262a8ac33d0aac07173d2b63c..803af159704f632102a65c6cea4b962b5d17524f 100644
--- a/sandbox/linux/services/credentials.cc
+++ b/sandbox/linux/services/credentials.cc
@@ -315,12 +315,16 @@ bool Credentials::DropFileSystemAccess(int proc_fd) {
   CHECK_LE(0, proc_fd);
 
   CHECK(ChrootToSafeEmptyDir());
-  CHECK(!base::DirectoryExists(base::FilePath("/proc")));
+  CHECK(!HasFileSystemAccess());
   CHECK(!ProcUtil::HasOpenDirectory(proc_fd));
   // We never let this function fail.
   return true;
 }
 
+bool Credentials::HasFileSystemAccess() {
+  return base::DirectoryExists(base::FilePath("/proc"));
+}
+
 pid_t Credentials::ForkAndDropCapabilitiesInChild() {
   pid_t pid = fork();
   if (pid != 0) {