Properly initialize last_notification_severity_.

BUG=622590
TEST=./out/Debug/unit_tests --gtest_filter=LowDiskNotificationTest.*

Review-Url: https://codereview.chromium.org/2115793002
Cr-Commit-Position: refs/heads/master@{#403420}

chrome/browser/chromeos/ui/low_disk_notification.h

@@ -61,7 +61,7 @@ class LowDiskNotification {
   void SetNotificationIntervalForTest(base::TimeDelta interval);
 
   base::Time last_notification_time_;
-  Severity last_notification_severity_;
+  Severity last_notification_severity_ = NONE;
   message_center::MessageCenter* message_center_;
   base::TimeDelta notification_interval_;
   base::ThreadChecker thread_checker_;