Update the libmetrics API to match the new Chrome interface.

src/platform/metrics/metrics_library.h

 // Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /*
  * metrics_library.h
  *
  *  Created on: Dec 1, 2009
  *      Author: sosa
  */
 
 #ifndef METRICS_LIBRARY_H_
 #define METRICS_LIBRARY_H_
 
 #include <string>
 
 // TODO(sosa@chromium.org): Add testing for send methods
 
-// Library used to send metrics both Autotest and Chrome
+// Library used to send metrics both Autotest and Chrome.
 class MetricsLibrary {
  public:
-  // Sends histogram data to Chrome and returns true on success.
-  static bool SendToChrome(std::string name, int value);
+  // Sends histogram data to Chrome for transport to UMA and returns
+  // true on success. This method results in the equivalent of an
+  // asynchronous non-blocking RPC to UMA_HISTOGRAM_CUSTOM_COUNTS
+  // inside Chrome (see base/histogram.h).
+  //
+  // |sample| is the sample value to be recorded (|min| <= |sample| < |max|).
+  // |min| is the minimum value of the histogram samples (|min| > 0).
+  // |max| is the maximum value of the histogram samples.
+  // |nbuckets| is the number of histogram buckets.
+  // [0,min) is the implicit underflow bucket.
+  // [|max|,infinity) is the implicit overflow bucket.
+  static bool SendToChrome(const std::string& name, int sample,
+                           int min, int max, int nbuckets);
+
   // Sends to Autotest and returns true on success.
-  static bool SendToAutotest(std::string name, int value);
+  static bool SendToAutotest(const std::string& name, int value);
 };
 
 #endif /* METRICS_LIBRARY_H_ */