Don't crash if MojoNetLog::ShutDown() is called twice.

Bug: 880608
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: I065803782935f7e69310a791ecb3053d573fcc61
Reviewed-on: https://chromium-review.googlesource.com/1205534Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588881}

services/network/mojo_net_log.cc

@@ -28,6 +28,7 @@ void MojoNetLog::ShutDown() {
   if (file_net_log_observer_) {
     file_net_log_observer_->StopObserving(nullptr /*polled_data*/,
                                           base::OnceClosure());
+    file_net_log_observer_.reset();
   }
 }
 

services/network/mojo_net_log.h

@@ -30,6 +30,7 @@ class MojoNetLog : public net::NetLog {
   ~MojoNetLog() override;
 
   // Finalizes the logfile created by any call to ObserveFileWithConstants().
+  // It is safe to call this multiple times.
   void ShutDown();
 
   // If specified by the command line, stream network events (NetLog) to a