Clean up histogram'd media enum max values.

media/base/sample_format.h

 // Copyright 2013 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 MEDIA_BASE_SAMPLE_FORMAT_H
 #define MEDIA_BASE_SAMPLE_FORMAT_H
 
 #include "media/base/media_export.h"
 
 namespace media {
 
 enum SampleFormat {
   // These values are histogrammed over time; do not change their ordinal
   // values.  When deleting a sample format replace it with a dummy value; when
-  // adding a sample format, do so at the bottom before kSampleFormatMax.
+  // adding a sample format, do so at the bottom before kSampleFormatMax, and
+  // update the value of kSampleFormatMax.
   kUnknownSampleFormat = 0,
   kSampleFormatU8,         // Unsigned 8-bit w/ bias of 128.
   kSampleFormatS16,        // Signed 16-bit.
   kSampleFormatS32,        // Signed 32-bit.
   kSampleFormatF32,        // Float 32-bit.
   kSampleFormatPlanarS16,  // Signed 16-bit planar.
   kSampleFormatPlanarF32,  // Float 32-bit planar.
 
-  // Must always be last!
-  kSampleFormatMax
+  // Must always be equal to largest value ever logged.
+  kSampleFormatMax = kSampleFormatPlanarF32,
 };
 
 // Returns the number of bytes used per channel for the specified
 // |sample_format|.
 MEDIA_EXPORT int SampleFormatToBytesPerChannel(SampleFormat sample_format);
 
 // Returns the name of the sample format as a string
 MEDIA_EXPORT const char* SampleFormatToString(SampleFormat sample_format);
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_SAMPLE_FORMAT_H