metrics: Add leak detector controller in Chrome OS metrics system

components/metrics/leak_detector/leak_detector.gni

+# Copyright 2016 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.
+
+# This is the GN equivalent of
+# //components/metrics/leak_detector/leak_detector.gypi.
+# Please keep in sync.
+
+declare_args() {
+  # Build runtime memory leak detector as part of Chrome. Enabling this without
+  # tuning its input parameters on may incur substantial CPU and memory
+  # overhead, as it is intended to be invoked during heap allocations and frees.
+  # It also requires Chrome to be compiled with frame pointer in order to work.
+  enable_leak_detector = false
+}