Record swap metrics

content/browser/memory/swap_metrics_observer_win.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 "content/browser/memory/swap_metrics_observer.h"
+
+namespace content {
+
+// static
+SwapMetricsObserver* SwapMetricsObserver::GetInstance() {
+  // SwapMetricsObserver isn't available on Windows.

[bashi, 2017/04/18 07:12:33]

I couldn't find any API to get swap rates and/or (de)compression rates on
Windows.

[Primiano Tucci (use gerrit), 2017/04/27 10:01:20]

There is a PageFaultsCount, PROCESS_MEMORY_COUNTERS_EX although I suspect that
would count soft + hard, while you want only hard here?
An alternative would be using WMI, but that is a huge pain and not sure it's
worth it here.
I'd ask chrisha@ / brucedawson@

[bashi, 2017/04/28 02:05:35]

Yeah, I thought PageFaultsCount may be noisy as it may count soft + hard. Also I
think that hard page faults would contain demand paging which doesn't necessary
to be considered as swap rates IMO.

I added a TODO comment to seek whether we can add swap rates on Windows.

+  return nullptr;
+}
+
+}  // namespace content