bug 1366294 - Part 11 - Fix OSX Build. r=gfritzsche

MozReview-Commit-ID: DSljXr8ZRhy

ipc/chromium/src/base/histogram.h

@@ -455,7 +455,7 @@ class CustomHistogram : public Histogram {
   virtual ClassType histogram_type() const;
 
  protected:
-  CustomHistogram(const std::vector<Sample>& custom_ranges);
+  explicit CustomHistogram(const std::vector<Sample>& custom_ranges);
 
   // Initialize ranges_ mapping.
   void InitializedCustomBucketRange(const std::vector<Sample>& custom_ranges);