Add UMA Histogram Manager to Cronet.

components/metrics/histogram_encoder.h

Index: components/metrics/histogram_encoder.h
diff --git a/components/metrics/histogram_encoder.h b/components/metrics/histogram_encoder.h
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+++ b/components/metrics/histogram_encoder.h
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+// Copyright 2014 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.
+
+// This file defines an utility function that records any changes in a given
+// histogram for transmission.
+
+#ifndef COMPONENTS_METRICS_HISTOGRAM_ENCODER_H_
+#define COMPONENTS_METRICS_HISTOGRAM_ENCODER_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+#include "components/metrics/proto/chrome_user_metrics_extension.pb.h"
+
+namespace base {
+class HistogramSamples;
+}
+
+namespace metrics {
+
+// Record any changes in a given histogram for transmission.
+void RecordHistogramDelta(const std::string& histogram_name,
+                          const base::HistogramSamples& snapshot,
+                          ChromeUserMetricsExtension* uma_proto);
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_HISTOGRAM_ENCODER_H_