dbus: revamp fd passing for i/o restrictions

dbus/file_descriptor.cc

Index: dbus/file_descriptor.cc
diff --git a/dbus/file_descriptor.cc b/dbus/file_descriptor.cc
index fc240f552c634524fb619e52095305fd054c5807..91b089f657cf271e5240e05e24be4f1eca9b6452 100644
--- a/dbus/file_descriptor.cc
+++ b/dbus/file_descriptor.cc
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "base/logging.h"
 #include "base/platform_file.h"
 #include "dbus/file_descriptor.h"
 
@@ -12,4 +13,21 @@ FileDescriptor::~FileDescriptor() {
     base::ClosePlatformFile(value_);
 }
 
+int FileDescriptor::value() const {
+  CHECK(valid_);
+  return value_;
+}
+
+int FileDescriptor::TakeValue() {
+  CHECK(valid_);  // NB: check first so owner_ is unchanged if this triggers
+  owner_ = false;
+  return value_;
+}
+
+void FileDescriptor::CheckValidity() {
+  base::PlatformFileInfo info;
+  bool ok = base::GetPlatformFileInfo(value_, &info);
+  valid_ = (ok && !info.is_directory);
+}
+
 }  // namespace dbus