Modify rappor parameters.

components/rappor/proto/rappor_metric.proto

 // Copyright 2014 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.
 //
 // Contains information collected by the RAPPOR (Randomized Aggregatable
 // Privacy-Preserving Ordinal Responses) system.
 //
 // For a full description of the rappor metrics, see
 // http://www.chromium.org/developers/design-documents/rappor
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package rappor;
 
 // Next tag: 3
 message RapporReports {
   // Which cohort these reports belong to.  The RAPPOR participants are
   // partioned into cohorts in different ways, to allow better statistics and
   // increased coverage.  In particular, the cohort will serve to choose the
   // hash functions used for Bloom-filter-based reports.  The cohort is
-  // generated randomly by the client and is currently in the range [0,32).
+  // generated randomly by the client and is currently in the range [0,512).
+  // Was in range [0,32) in chrome versions before M37.
   optional int32 cohort = 1;
 
   // Each Report contains the values generated by the RAPPOR process for one
   // metric.
   message Report {
     // The name of the metric, hashed (first 8 bytes of MD5 hash).
     optional fixed64 name_hash = 1;
 
     // The sequence of bits produced by random coin flips in
     // RapporMetric::GetReport().  For a complete description of RAPPOR
     // metrics, refer to the design document at:
     // http://www.chromium.org/developers/design-documents/rappor
     optional bytes bits = 2;
   }
   repeated Report report = 2;
 }