Randomly select a single rappor sample when more than one is collected.

components/rappor/rappor_metric.cc

Index: components/rappor/rappor_metric.cc
diff --git a/components/rappor/rappor_metric.cc b/components/rappor/rappor_metric.cc
index b4bd950af9d1741210b81dcdf54bbd893b1b63b2..92765de6e89b79e7c3c02873c2c6403b53fb2fea 100644
--- a/components/rappor/rappor_metric.cc
+++ b/components/rappor/rappor_metric.cc
@@ -5,6 +5,7 @@
 #include "components/rappor/rappor_metric.h"
 
 #include "base/logging.h"
+#include "base/rand_util.h"
 
 namespace rappor {
 
@@ -13,6 +14,7 @@ RapporMetric::RapporMetric(const std::string& metric_name,
                            int32_t cohort_seed)
     : metric_name_(metric_name),
       parameters_(parameters),
+      sample_count_(0),
       bloom_filter_(parameters.bloom_filter_size_bytes,
                     parameters.bloom_filter_hash_function_count,
                     (cohort_seed % parameters.num_cohorts) *
@@ -24,7 +26,12 @@ RapporMetric::RapporMetric(const std::string& metric_name,
 RapporMetric::~RapporMetric() {}
 
 void RapporMetric::AddSample(const std::string& str) {
-  bloom_filter_.AddString(str);
+  ++sample_count_;
+  // Replace the previous sample with a 1 in sample_count_ chance so that each
+  // sample has equal probability of being reported.
+  if (base::RandGenerator(sample_count_) == 0) {
+    bloom_filter_.SetString(str);
+  }
 }
 
 ByteVector RapporMetric::GetReport(const std::string& secret) const {
@@ -59,4 +66,8 @@ ByteVector RapporMetric::GetReport(const std::string& secret) const {
   return *ByteVectorMerge(*fake_and_redacted_bits, zero_coins, &one_coins);
 }
 
+void RapporMetric::SetBytesForTesting(const ByteVector& bytes) {
+  bloom_filter_.SetBytesForTesting(bytes);
+}
+
 }  // namespace rappor