Add missing (and remove superfluous) 'explicit' from constructors.

base/metrics/sample_map.h

 // Copyright (c) 2012 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.
 
 // SampleMap implements HistogramSamples interface. It is used by the
 // SparseHistogram class to store samples.
 
 #ifndef BASE_METRICS_SAMPLE_MAP_H_
 #define BASE_METRICS_SAMPLE_MAP_H_
 
@@ skipping 30 matching lines @@
   std::map<HistogramBase::Sample, HistogramBase::Count> sample_counts_;
 
   DISALLOW_COPY_AND_ASSIGN(SampleMap);
 };
 
 class BASE_EXPORT_PRIVATE SampleMapIterator : public SampleCountIterator {
  public:
   typedef std::map<HistogramBase::Sample, HistogramBase::Count>
       SampleToCountMap;
 
-  SampleMapIterator(const SampleToCountMap& sample_counts);
+  explicit SampleMapIterator(const SampleToCountMap& sample_counts);
   virtual ~SampleMapIterator();
 
   // SampleCountIterator implementation:
   virtual bool Done() const OVERRIDE;
   virtual void Next() OVERRIDE;
   virtual void Get(HistogramBase::Sample* min,
                    HistogramBase::Sample* max,
                    HistogramBase::Count* count) const OVERRIDE;
  private:
   SampleToCountMap::const_iterator iter_;
   const SampleToCountMap::const_iterator end_;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_SAMPLE_MAP_H_