Fix iOSMainThreadFreeze metrics

The reporting of metrics has been broken by
https://chromium-review.googlesource.com/c/chromium/src/+/1405508

The SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN will now be reported
only if there is no normal crash report (main thread freeze report should
only be seen as an explanation of crashes without real report).

Bug: 1052377
Change-Id: I1aa17d5061735732324448128a005cea4f9d9ba6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060499Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742177}

ios/chrome/browser/crash_report/main_thread_freeze_detector.mm

@@ -75,6 +75,7 @@ void LogRecoveryTime(base::TimeDelta time) {
   if (self) {
     _lastSessionFreezeInfo = [[NSUserDefaults standardUserDefaults]
         dictionaryForKey:@(kNsUserDefaultKeyLastSessionInfo)];
+    _lastSessionEndedFrozen = _lastSessionFreezeInfo != nil;
     [[NSUserDefaults standardUserDefaults]
         removeObjectForKey:@(kNsUserDefaultKeyLastSessionInfo)];
     _delay = kFreezeDetectionDelay;

ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider.mm

@@ -202,23 +202,23 @@ MobileSessionShutdownMetricsProvider::GetLastShutdownType() {
     return SHUTDOWN_IN_BACKGROUND;
   }
 
-  // If the last app lifetime ended with main thread not responding, log it as
-  // main thread frozen shutdown.
-  if (LastSessionEndedFrozen()) {
-    return SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN;
-  }
-
-  // If the last app lifetime ended in a crash, log the type of crash.
-  if (ReceivedMemoryWarningBeforeLastShutdown()) {
   if (HasCrashLogs()) {
+    // The cause of the crash is known.
+    if (ReceivedMemoryWarningBeforeLastShutdown()) {
       return SHUTDOWN_IN_FOREGROUND_WITH_CRASH_LOG_WITH_MEMORY_WARNING;
     }
-    return SHUTDOWN_IN_FOREGROUND_NO_CRASH_LOG_WITH_MEMORY_WARNING;
+    return SHUTDOWN_IN_FOREGROUND_WITH_CRASH_LOG_NO_MEMORY_WARNING;
   }
 
-  if (HasCrashLogs()) {
-    return SHUTDOWN_IN_FOREGROUND_WITH_CRASH_LOG_NO_MEMORY_WARNING;
+  // The cause of the crash is not known. Check the common causes in order of
+  // severity and likeliness to have caused the crash.
+  if (LastSessionEndedFrozen()) {
+    return SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN;
+  }
+  if (ReceivedMemoryWarningBeforeLastShutdown()) {
+    return SHUTDOWN_IN_FOREGROUND_NO_CRASH_LOG_WITH_MEMORY_WARNING;
   }
+  // There is no known cause.
   return SHUTDOWN_IN_FOREGROUND_NO_CRASH_LOG_NO_MEMORY_WARNING;
 }
 

ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm

@@ -115,8 +115,8 @@ TEST_P(MobileSessionShutdownMetricsProviderTest, ProvideStabilityMetrics) {
       SHUTDOWN_IN_FOREGROUND_WITH_CRASH_LOG_WITH_MEMORY_WARNING,
       SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN,
       SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN,
-      SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN,
-      SHUTDOWN_IN_FOREGROUND_WITH_MAIN_THREAD_FROZEN,
+      SHUTDOWN_IN_FOREGROUND_WITH_CRASH_LOG_NO_MEMORY_WARNING,
+      SHUTDOWN_IN_FOREGROUND_WITH_CRASH_LOG_WITH_MEMORY_WARNING,
       // If wasLastShutdownClean is true, the memory warning and crash log don't
       // matter.
       SHUTDOWN_IN_BACKGROUND,