Avoid deadlock during trace_event flush

The deadlock occurs because we hold lock when posting flush tasks.
If MessageLoop::PostTask() also calls TRACE_EVENT and the TRACE_EVENT
has never been reached before, the TRACE_EVENT will try to get the
category enabled flag which will also need a lock.

Copy thread_message_loops into a temporary vector of
scoped_refptr<SingleProcessTaskRunner> to avoid lock when calling
PostTask().

BUG=397022
TEST=Existing tests and ThreadSanitizer

Review URL: https://codereview.chromium.org/491393002

Cr-Commit-Position: refs/heads/master@{#293144}

base/debug/trace_event_impl.cc

@@ -1697,6 +1697,9 @@ void TraceLog::Flush(const TraceLog::OutputCallback& cb) {
   }
 
   int generation = this->generation();
+  // Copy of thread_message_loops_ to be used without locking.
+  std::vector<scoped_refptr<SingleThreadTaskRunner> >
+      thread_message_loop_task_runners;
   {
     AutoLock lock(lock_);
     DCHECK(!flush_message_loop_proxy_.get());
@@ -1713,7 +1716,14 @@ void TraceLog::Flush(const TraceLog::OutputCallback& cb) {
       for (hash_set<MessageLoop*>::const_iterator it =
            thread_message_loops_.begin();
            it != thread_message_loops_.end(); ++it) {
-        (*it)->PostTask(
+        thread_message_loop_task_runners.push_back((*it)->task_runner());
+      }
+    }
+  }
+
+  if (thread_message_loop_task_runners.size()) {
+    for (size_t i = 0; i < thread_message_loop_task_runners.size(); ++i) {
+      thread_message_loop_task_runners[i]->PostTask(
           FROM_HERE,
           Bind(&TraceLog::FlushCurrentThread, Unretained(this), generation));
     }
@@ -1723,7 +1733,6 @@ void TraceLog::Flush(const TraceLog::OutputCallback& cb) {
         TimeDelta::FromMilliseconds(kThreadFlushTimeoutMs));
     return;
   }
-  }
 
   FinishFlush(generation);
 }