Fix Value::GetDouble() crash in SessionDurationUpdater

It looks like occasionally the kObservedSessionTime pref is corrupt or
otherwise incorrect, resulting in a crash upon a Value::GetDouble()
call on a value in the dictionary. This should fix the crash by
verifying the value type.

Bug: 942397
Change-Id: I15389c725dc61d96b12912dbc21ed9e69648852d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529719Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642274}

chrome/browser/feature_engagement/session_duration_updater.cc

@@ -40,9 +40,8 @@ base::TimeDelta SessionDurationUpdater::GetCumulativeElapsedSessionTime()
 base::TimeDelta SessionDurationUpdater::GetRecordedObservedSessionTime() const {
   const base::DictionaryValue* dict =
       pref_service_->GetDictionary(prefs::kObservedSessionTime);
-  const base::Value* dict_value =
-      dict->FindKey(observed_session_time_dict_key_);
-  const double stored_value = dict_value ? dict_value->GetDouble() : 0L;
+  const double stored_value =
+      dict->FindDoubleKey(observed_session_time_dict_key_).value_or(0);
   return base::TimeDelta::FromSeconds(stored_value);
 }
 
@@ -85,9 +84,8 @@ void SessionDurationUpdater::OnSessionEnded(base::TimeDelta elapsed) {
 
   const base::DictionaryValue* dict =
       pref_service_->GetDictionary(prefs::kObservedSessionTime);
-  const base::Value* dict_value =
-      dict->FindKey(observed_session_time_dict_key_);
-  const double stored_value = dict_value ? dict_value->GetDouble() : 0L;
+  const double stored_value =
+      dict->FindDoubleKey(observed_session_time_dict_key_).value_or(0);
 
   base::TimeDelta elapsed_session_time =
       base::TimeDelta::FromSeconds(stored_value) + elapsed;