Fix race over RtcEventLogImpl::task_queue_

RtcEventLogImpl::task_queue_ is a std::unique_ptr<rtc::TaskQueue>.
When a unique_ptr is destroyed, it first sets its internal pointer
to point to null, and only then invokes the destructor of that
object. However, the code in RtcEventLogImpl relies on
rtc::TaskQueue's property, that its destructor blocks on executing
tasks.

We solve by manually invoking the destructor, and only resetting
the internal pointer thereafter. In theory, we could have changed
the unique_ptr to a raw pointer at this point. We avoid that, so
as to keep the ownership clearer to readers of the code.

Bug: webrtc:10085
Change-Id: I54bbf5d6bae019757ca2e31ee960d558058ccc42
Reviewed-on: https://webrtc-review.googlesource.com/c/112598
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25875}

logging/rtc_event_log/rtc_event_log_impl.cc

@@ -159,6 +159,12 @@ RtcEventLogImpl::~RtcEventLogImpl() {
 
   // If we're logging to the output, this will stop that. Blocking function.
   StopLogging();
+
+  // We want to block on any executing task by invoking ~TaskQueue() before
+  // we set unique_ptr's internal pointer to null.
+  rtc::TaskQueue* tq = task_queue_.get();
+  delete tq;
+  task_queue_.release();
 }
 
 bool RtcEventLogImpl::StartLogging(std::unique_ptr<RtcEventLogOutput> output,