Add UMA metrics for root scroller intervention to track forcing passive breakage.

third_party/WebKit/Source/platform/testing/HistogramTester.h

 // 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.
 
 #ifndef HistogramTester_h
 #define HistogramTester_h
 
 #include "platform/Histogram.h"
 #include <memory>
 
 namespace base {
 class HistogramTester;
 }
 
 namespace blink {
 
 // Blink interface for base::HistogramTester.
 class HistogramTester {
 public:
     HistogramTester();
     ~HistogramTester();
 
     void expectUniqueSample(const std::string& name, base::HistogramBase::Sample, base::HistogramBase::Count) const;
     void expectTotalCount(const std::string& name, base::HistogramBase::Count) const;
+    base::HistogramBase::Count histogramCount(const std::string& name, base::HistogramBase::Sample) const;

[Ilya Sherman, 2016/07/12 21:26:18]

I'd prefer to keep this class analogous to base::HistogramTester, and hence
expose this functionality as expectBucketCount().

 
 private:
     std::unique_ptr<base::HistogramTester> m_histogramTester;
 };
 
 } // namespace blink
 
 #endif // HistogramTester_h