Support caching histograms so that a lookup isn't done in each iteration.

third_party/WebKit/Source/platform/Histogram.cpp

+// Copyright 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 "platform/Histogram.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/sparse_histogram.h"
+
+namespace blink {
+
+CustomCountHistogram::CustomCountHistogram(const char* name, int32_t min, int32_t max, int32_t bucketCount)
+{
+    m_histogram = base::Histogram::FactoryGet(name, min, max, bucketCount, base::HistogramBase::kNoFlags);

[Alexei Svitkine (slow), 2016/02/01 19:55:31]

I think you want kUmaTargetedHistogramFlag here and on line 28, else these
histograms won't be sent up to UMA server.

+}
+
+CustomCountHistogram::CustomCountHistogram(base::HistogramBase* histogram)
+    : m_histogram(histogram)
+{
+}
+
+void CustomCountHistogram::count(int32_t sample)
+{
+    m_histogram->Add(sample);
+}
+
+EnumerationHistogram::EnumerationHistogram(const char* name, int32_t boundaryValue)
+    : CustomCountHistogram(base::LinearHistogram::FactoryGet(name, 1, boundaryValue, boundaryValue + 1, base::HistogramBase::kNoFlags))
+{
+}
+
+SparseHistogram::SparseHistogram(const char* name)
+{
+    m_histogram = base::SparseHistogram::FactoryGet(name, base::HistogramBase::kUmaTargetedHistogramFlag);
+}
+
+void SparseHistogram::sample(int32_t sample)
+{
+    m_histogram->Add(sample);
+}
+
+} // namespace blink