Add UMA protocol buffers to Chrome.

chrome/common/metrics/proto/histogram_event.proto

+// 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.
+//
+// Histogram-collected metrics.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package metrics;
+
+// Next tag: 4
+message HistogramEventProto {
+  // The name of the histogram, hashed.
+  optional fixed64 name_hash = 1;
+
+  // The sum of all the sample values.
+  // Together with the total count of the sample values, this allows us to
+  // compute the average value.  The count of all sample values is just the sum
+  // of the counts of all the buckets.
+  optional int64 sum = 2;
+
+  // The per-bucket data.
+  message Bucket {
+    // Each bucket's range is bounded by min <= x < max.
+    // We expect min and max (as well all other fields in this file) to always
+    // be set.  They're marked as optional because that is considered to be good
+    // practice in protocol buffer design, for the sake of
+    // forward-compatibility.
+    optional int64 min = 1;
+    optional int64 max = 2;
+
+    // The bucket's index in the list of buckets, sorted in ascending order.
+    // Historically, we've had trouble with corruption of the min or the max,
+    // most commonly in the client.  The bucket index gives us some redundancy,
+    // which allows the processing code to "vote" on what the correct range
+    // should be for each bucket.  This in turn allows us to better identify and
+    // discard corrupted reports.
+    optional int32 bucket_index = 3;
+
+    // The number of entries in this bucket.
+    optional int64 count = 4;
+  }
+  repeated Bucket bucket = 3;
+}