Added MemoryMonitor for Linux (with test)

components/memory_coordinator/browser/memory_monitor.cc

+// Copyright (c) 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.
+
+#include "components/memory_coordinator/browser/memory_monitor.h"
+
+#include "base/process/process_metrics.h"
+
+namespace memory_coordinator {
+
+MemoryMonitorDelegate::~MemoryMonitorDelegate() {}
+
+void MemoryMonitorDelegate::GetSystemMemoryInfo(
+    base::SystemMemoryInfoKB* mem_info) {
+  base::GetSystemMemoryInfo(mem_info);
+}
+
+}  // namespace memory_coordinator