[Chromecast] Destruct task runner after cma backend

CmaBackend still uses task runner in its destructor.

Bug: internal b/150796363
Test: Build
Change-Id: Idfa283dd7f72d06243ba4ab5803971d33035e5c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097325
Auto-Submit: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748974}

chromecast/media/audio/cma_audio_output_stream.cc

@@ -193,9 +193,9 @@ void CmaAudioOutputStream::Close(base::OnceClosure closure) {
   source_callback_ = nullptr;
   cma_backend_state_ = CmaBackendState::kPendingClose;
 
-  cma_backend_task_runner_.reset();
-  cma_backend_.reset();
   audio_bus_.reset();
+  cma_backend_.reset();
+  cma_backend_task_runner_.reset();
 
   std::move(closure).Run();
 }