Use low entropy for studies that send experiment IDs to Google properties.

components/metrics/metrics_state_manager.cc

Index: components/metrics/metrics_state_manager.cc
diff --git a/components/metrics/metrics_state_manager.cc b/components/metrics/metrics_state_manager.cc
index 0f2904e5ff036be29b872b38c05fb888402f7ee8..e71264e3912a94743555b031e07077a2b4fb4e83 100644
--- a/components/metrics/metrics_state_manager.cc
+++ b/components/metrics/metrics_state_manager.cc
@@ -43,6 +43,12 @@ int GenerateLowEntropySource() {
   return base::RandInt(0, kMaxLowEntropySize - 1);
 }
 
+// Records the given |low_entorpy_source_value| in a histogram.
+void LogLowEntropyValue(int low_entropy_source_value) {
+  UMA_HISTOGRAM_SPARSE_SLOWLY("UMA.LowEntropySourceValue",
+                              low_entropy_source_value);
+}
+
 }  // namespace
 
 // static
@@ -148,7 +154,7 @@ void MetricsStateManager::CheckForClonedInstall(
 }
 
 std::unique_ptr<const base::FieldTrial::EntropyProvider>
-MetricsStateManager::CreateEntropyProvider() {
+MetricsStateManager::CreateDefaultEntropyProvider() {
   // For metrics reporting-enabled users, we combine the client ID and low
   // entropy source to get the final entropy source. Otherwise, only use the low
   // entropy source.
@@ -157,8 +163,6 @@ MetricsStateManager::CreateEntropyProvider() {
   //     know the low entropy source.
   //  2) It makes the final entropy source resettable.
   const int low_entropy_source_value = GetLowEntropySource();

[Alexei Svitkine (slow), 2016/05/18 15:03:30]

Nit: Move this inside the if, so that it's not done redundantly if the if is
false.

[jwd, 2016/05/18 18:31:41]

Done.

-  UMA_HISTOGRAM_SPARSE_SLOWLY("UMA.LowEntropySourceValue",
-                              low_entropy_source_value);
   if (enabled_state_provider_->IsConsentGiven()) {
     UpdateEntropySourceReturnedValue(ENTROPY_SOURCE_HIGH);
     const std::string high_entropy_source =
@@ -168,6 +172,13 @@ MetricsStateManager::CreateEntropyProvider() {
   }
 
   UpdateEntropySourceReturnedValue(ENTROPY_SOURCE_LOW);
+  return CreateLowEntropyProvider();
+}
+
+std::unique_ptr<const base::FieldTrial::EntropyProvider>
+MetricsStateManager::CreateLowEntropyProvider() {
+  const int low_entropy_source_value = GetLowEntropySource();
+
 #if defined(OS_ANDROID) || defined(OS_IOS)
   return std::unique_ptr<const base::FieldTrial::EntropyProvider>(
       new CachingPermutedEntropyProvider(local_state_, low_entropy_source_value,
@@ -198,8 +209,6 @@ std::unique_ptr<MetricsStateManager> MetricsStateManager::Create(
 // static
 void MetricsStateManager::RegisterPrefs(PrefRegistrySimple* registry) {
   registry->RegisterBooleanPref(prefs::kMetricsResetIds, false);
-  // registry->RegisterIntegerPref(prefs::kMetricsDefaultOptIn,
-  // DEFAULT_UNKNOWN);
   registry->RegisterStringPref(prefs::kMetricsClientID, std::string());
   registry->RegisterInt64Pref(prefs::kMetricsReportingEnabledTimestamp, 0);
   registry->RegisterIntegerPref(prefs::kMetricsLowEntropySource,
@@ -274,11 +283,13 @@ void MetricsStateManager::UpdateLowEntropySource() {
     // it below.
     if (value >= 0 && value < kMaxLowEntropySize) {
       low_entropy_source_ = value;
+      LogLowEntropyValue(low_entropy_source_);
       return;
     }
   }
 
   low_entropy_source_ = GenerateLowEntropySource();
+  LogLowEntropyValue(low_entropy_source_);
   local_state_->SetInteger(prefs::kMetricsLowEntropySource,
                            low_entropy_source_);
   CachingPermutedEntropyProvider::ClearCache(local_state_);