Make GlobalDescriptors::MaybeGet return -1 when the key is not found.

ipc/ipc_multiprocess_test.cc

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "build/build_config.h"
+
+#include "ipc/ipc_multiprocess_test.h"
+
+#if defined(OS_POSIX)
+#include "base/global_descriptors_posix.h"
+#include "ipc/ipc_descriptors.h"
+#endif
+
+void MultiProcessTestIPCSetUp() {
+#if defined(OS_POSIX)
+  base::GlobalDescriptors::GetInstance()->Set(kPrimaryIPCChannel,
+      kPrimaryIPCChannel + base::GlobalDescriptors::kBaseDescriptor);
+#endif
+}