Enable clang warning for ignored attributes.

base/metrics/histogram_base.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.
 
 #ifndef BASE_METRICS_HISTOGRAM_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 13 matching lines @@
 class HistogramSamples;
 class ListValue;
 class Pickle;
 class PickleIterator;
 
 ////////////////////////////////////////////////////////////////////////////////
 // These enums are used to facilitate deserialization of histograms from other
 // processes into the browser. If you create another class that inherits from
 // HistogramBase, add new histogram types and names below.
 
-enum BASE_EXPORT HistogramType {
+enum HistogramType {
   HISTOGRAM,
   LINEAR_HISTOGRAM,
   BOOLEAN_HISTOGRAM,
   CUSTOM_HISTOGRAM,
   SPARSE_HISTOGRAM,
 };
 
 std::string HistogramTypeToString(HistogramType type);
 
 // Create or find existing histogram that matches the pickled info.
@@ skipping 130 matching lines @@
  private:
   const std::string histogram_name_;
   int32_t flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_