Fix UKM crash. Add TODO for fixing this properly.

components/ukm/ukm_service.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/ukm/ukm_service.h"
 
 #include <memory>
 #include <string>
 #include <utility>
 
@@ skipping 390 matching lines @@
   bool discard_log = false;
   const size_t log_size_bytes = persisted_logs_.staged_log().length();
   if (upload_succeeded) {
     UMA_HISTOGRAM_COUNTS_10000("UKM.LogSize.OnSuccess", log_size_bytes / 1024);
   } else if (response_code == 400) {
     // Bad syntax.  Retransmission won't work.
     discard_log = true;
   }
 
   if (upload_succeeded || discard_log) {
-    persisted_logs_.DiscardStagedLog();
+    // TODO(holte): The if below is a temporary fix for a crash bug. We should
+    // revisit the logic and update it with a more correct fix. crbug.com/698819
+    if (persisted_logs_.has_staged_log())
+      persisted_logs_.DiscardStagedLog();
     // Store the updated list to disk now that the removed log is uploaded.
     persisted_logs_.PersistUnsentLogs();
   }
 
   // Error 400 indicates a problem with the log, not with the server, so
   // don't consider that a sign that the server is in trouble.
   bool server_is_healthy = upload_succeeded || response_code == 400;
   scheduler_->UploadFinished(server_is_healthy,
                              persisted_logs_.has_unsent_logs());
 }
@@ skipping 65 matching lines @@
   }
   if (entries_.size() >= GetMaxEntries()) {
     RecordDroppedEntry(DroppedDataReason::MAX_HIT);
     return;
   }
 
   entries_.push_back(std::move(entry));
 }
 
 }  // namespace ukm