[Chromecast] Remove false DCHECK in CastAudioOutputStream.

Given |GetMaxBufferedFrames| runs during attribute initialization, the
thread checker this DCHECK looks for doesn't exist yet causing crashes.

This check is unnecessary as the only member variables that are accessed
by the method are const.

Bug: internal 173625018
Test: Build eng build and check no crash.
Change-Id: I34e8b1b51ba75a3325466c1383a7000f039463b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553221Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Adam Whiteside <awhiteside@google.com>
Auto-Submit: Adam Whiteside <awhiteside@google.com>
Cr-Commit-Position: refs/heads/master@{#829893}

chromecast/media/audio/cast_audio_output_stream.cc

@@ -212,7 +212,6 @@ void CastAudioOutputStream::MixerServiceWrapper::Close(
 }
 
 int64_t CastAudioOutputStream::MixerServiceWrapper::GetMaxBufferedFrames() {
-  DCHECK_CALLED_ON_VALID_THREAD(io_thread_checker_);
   int fill_size_frames = audio_params_.frames_per_buffer();
   base::TimeDelta target_max_buffered_ms = kMediaMaxBufferedFrames;
   if (GetContentType(device_id_) == AudioContentType::kCommunication) {