Add support for single sample metrics.

base/metrics/single_value_histograms.cc

+// Copyright 2017 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 "base/metrics/single_value_histograms.h"
+
+namespace base {
+
+static SingleValueHistogramsFactory* g_factory = nullptr;
+
+// TODO(dalecurtis): What to do about tests that don't have a ChildThread? Now
+// we'd need all run_all_unittests.cc to set this up, which is annoying...
+
+// static
+SingleValueHistogramsFactory* SingleValueHistogramsFactory::Get() {
+  return g_factory;
+}
+
+// static
+void SingleValueHistogramsFactory::SetFactory(
+    SingleValueHistogramsFactory* factory) {
+  DCHECK(!g_factory);
+  g_factory = factory;
+}
+
+}  // namespace base