Reset some UMA stability stats on a version change.

components/metrics/metrics_service.cc

Index: components/metrics/metrics_service.cc
diff --git a/components/metrics/metrics_service.cc b/components/metrics/metrics_service.cc
index 371eac6ea33ef55a2bdff965d5078a4a1c70ca1e..94d3d5376adee657d1ab6ad646f1db11f15f5dd1 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -558,10 +558,15 @@ void MetricsService::RecordBreakpadHasDebugger(bool has_debugger) {
 // Initialization methods
 
 void MetricsService::InitializeMetricsState() {
-  local_state_->SetString(metrics::prefs::kStabilityStatsVersion,
-                          client_->GetVersionString());
-  local_state_->SetInt64(metrics::prefs::kStabilityStatsBuildTime,
-                         MetricsLog::GetBuildTime());
+  const int64 buildtime = MetricsLog::GetBuildTime();
+  const std::string version = client_->GetVersionString();
+  bool version_changed = false;
+  if (local_state_->GetInt64(prefs::kStabilityStatsBuildTime) != buildtime ||
+      local_state_->GetString(prefs::kStabilityStatsVersion) != version) {
+    local_state_->SetString(metrics::prefs::kStabilityStatsVersion, version);
+    local_state_->SetInt64(metrics::prefs::kStabilityStatsBuildTime, buildtime);
+    version_changed = true;
+  }
 
   log_manager_.LoadPersistedUnsentLogs();
 
@@ -589,6 +594,21 @@ void MetricsService::InitializeMetricsState() {
       PrepareInitialStabilityLog();
   }
 
+  // If no initial stability log was generated and there was a version upgrade,
+  // reset stability stats from the previous version.

[Ilya Sherman, 2014/09/18 18:30:20]

nit: I'd go into a bit more detail here on why this might happen -- something
like "There is an edge case where the initial stability log can fail to be
generated even if the previous version crashed: It's possible that the previous
version crashed before it was able to save off a system profile. In this case,
we can't correctly identify what version the saved stability metrics correspond
to; but since we detected that the version changed, we at least know that it's
not the current version."

... but, you know, shorter would be better.  Please improve that further; I just
think the general idea is important to get across.

[Alexei Svitkine (slow), 2014/09/18 20:21:14]

Done.

+  if (!has_initial_stability_log_ && version_changed) {
+    for (size_t i = 0; i < metrics_providers_.size(); ++i)
+      metrics_providers_[i]->ClearSavedStabilityMetrics();
+
+    // Reset the prefs that are managed by MetricsService/MetricsLog directly.
+    local_state_->SetInteger(prefs::kStabilityCrashCount, 0);
+    local_state_->SetInteger(prefs::kStabilityExecutionPhase,
+                             UNINITIALIZED_PHASE);
+    local_state_->SetInteger(prefs::kStabilityIncompleteSessionEndCount, 0);
+    local_state_->SetInteger(prefs::kStabilityLaunchCount, 0);
+    local_state_->SetBoolean(prefs::kStabilitySessionEndCompleted, true);
+  }
+
   // Update session ID.
   ++session_id_;
   local_state_->SetInteger(metrics::prefs::kMetricsSessionID, session_id_);