Extract the handling of the clean exit beacon from MetricsService. The extracted implementation sup…

components/metrics/metrics_pref_names.cc

Index: components/metrics/metrics_pref_names.cc
diff --git a/components/metrics/metrics_pref_names.cc b/components/metrics/metrics_pref_names.cc
index c6d12eda075bae3bf66dfb0da4741a4f54f26917..5b87cfe9840b2be4d404eae23c5e3fc4dc868d63 100644
--- a/components/metrics/metrics_pref_names.cc
+++ b/components/metrics/metrics_pref_names.cc
@@ -83,10 +83,6 @@ const char kStabilityDebuggerNotPresent[] =
 const char kStabilityExecutionPhase[] =
     "user_experience_metrics.stability.execution_phase";
 
-// True if the previous run of the program exited cleanly.
-const char kStabilityExitedCleanly[] =
-    "user_experience_metrics.stability.exited_cleanly";
-
 // Number of times the session end did not complete.
 const char kStabilityIncompleteSessionEndCount[] =
     "user_experience_metrics.stability.incomplete_session_end_count";