Add uma stats to help evaluate the impact of changes to the quota allocation logic.

storage/common/storage_histograms.cc

Index: storage/common/storage_histograms.cc
diff --git a/storage/common/storage_histograms.cc b/storage/common/storage_histograms.cc
new file mode 100644
index 0000000000000000000000000000000000000000..fdae1a63358e2789f04a064e5037b0fbd654c159
--- /dev/null
+++ b/storage/common/storage_histograms.cc
@@ -0,0 +1,21 @@
+// 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 "storage/common/storage_histograms.h"
+
+#include "base/metrics/histogram_functions.h"
+
+namespace storage {
+
+void RecordBytesWritten(const char* label, int amount) {
+  const std::string name = "Storage.BytesWritten.";
+  base::UmaHistogramCounts10M(name + label, amount);
+}
+
+void RecordBytesRead(const char* label, int amount) {
+  const std::string name = "Storage.BytesRead.";
+  base::UmaHistogramCounts10M(name + label, amount);
+}
+
+}  // namespace storage