Added memory usage function to base.

webrtc/base/memory_usage_unittest.cc

+/*
+ *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/base/memory_usage.h"
+#include <cstdio>
+#include "webrtc/test/gtest.h"
+
+namespace rtc {
+
+TEST(GetMemoryUsage, SimpleTest) {
+  int64_t used_bytes = GetProcessResidentSizeBytes();
+  EXPECT_GE(used_bytes, 0);
+}
+
+}  // namespace rtc
+