Fix race causing crash in net::FileNetLogObserver shutdown

net/log/file_net_log_observer.cc

Index: net/log/file_net_log_observer.cc
diff --git a/net/log/file_net_log_observer.cc b/net/log/file_net_log_observer.cc
index 721ae8744ccfebe53f58ea680ca4f6925127b4dd..0c05f9e1e2f3a239090d9e3024231a09b44b343e 100644
--- a/net/log/file_net_log_observer.cc
+++ b/net/log/file_net_log_observer.cc
@@ -265,10 +265,10 @@ std::unique_ptr<FileNetLogObserver> FileNetLogObserver::CreateUnbounded(
 FileNetLogObserver::~FileNetLogObserver() {
   if (net_log()) {
     // StopObserving was not called.

[mmenke, 2017/06/08 00:45:53]

Maybe just call StopObserving with an empty callback instead?

[pauljensen, 2017/06/08 00:48:57]

This code posts DeleteAllFiles() while the other posts FlushThenStop(), so I'm
not sure they're interchangeable.

+    net_log()->DeprecatedRemoveObserver(this);
     file_task_runner_->PostTask(
         FROM_HERE, base::Bind(&FileNetLogObserver::FileWriter::DeleteAllFiles,
                               base::Unretained(file_writer_)));
-    net_log()->DeprecatedRemoveObserver(this);
   }
   file_task_runner_->DeleteSoon(FROM_HERE, file_writer_);
 }
@@ -280,13 +280,13 @@ void FileNetLogObserver::StartObserving(NetLog* net_log,
 
 void FileNetLogObserver::StopObserving(std::unique_ptr<base::Value> polled_data,
                                        const base::Closure& callback) {
+  net_log()->DeprecatedRemoveObserver(this);
+
   file_task_runner_->PostTaskAndReply(
       FROM_HERE, base::Bind(&FileNetLogObserver::FileWriter::FlushThenStop,
                             base::Unretained(file_writer_), write_queue_,
                             base::Passed(&polled_data)),
       callback);
-
-  net_log()->DeprecatedRemoveObserver(this);
 }
 
 void FileNetLogObserver::OnAddEntry(const NetLogEntry& entry) {
@@ -437,6 +437,7 @@ void FileNetLogObserver::BoundedFileWriter::Flush(
   write_queue->SwapQueue(&local_file_queue);
 
   std::string to_print;
+  CHECK(!event_files_.empty());
   size_t file_size = ftell(event_files_[current_file_idx_].get());
   size_t memory_freed = 0;