[Ozone-Drm] Do not try to take ref_ptr from destructor

Can't post a task with a ref-counted instance of the watcher from the
destructor.

BUG=467170

Review URL: https://codereview.chromium.org/1006403002

Cr-Commit-Position: refs/heads/master@{#320739}

ui/ozone/platform/drm/gpu/drm_device.cc

@@ -114,6 +114,7 @@ class DrmDevice::IOWatcher
     base::WaitableEvent done(false, false);
     io_task_runner_->PostTask(
         FROM_HERE, base::Bind(&IOWatcher::SetPausedOnIO, this, &done));
+    done.Wait();
   }
 
   void Shutdown() {
@@ -125,7 +126,7 @@ class DrmDevice::IOWatcher
  private:
   friend class base::RefCountedThreadSafe<IOWatcher>;
 
-  ~IOWatcher() override { SetPaused(true); }
+  ~IOWatcher() override {}
 
   void RegisterOnIO() {
     DCHECK(base::MessageLoopForIO::IsCurrent());