[Chromecast] Fix mixer's "ready to play" for first stream

If the mixer didn't have rendering delay available when the start
threshold was exceeded, the "ready to play" signal would never be sent.
Fix by ignoring rendering delay status.

Bug: internal b/158038278
Change-Id: I196be77210ea879ae673b704499f1c683ade8dc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342305Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795967}

chromecast/media/cma/backend/mixer/mixer_input_connection.cc

@@ -596,8 +596,7 @@ int64_t MixerInputConnection::QueueData(scoped_refptr<net::IOBuffer> data) {
     queued_frames_ += frames;
     queue_.push_back(std::move(data));
 
-    if (!started_ && queued_frames_ >= start_threshold_frames_ &&
-        mixer_rendering_delay_.timestamp_microseconds != INT64_MIN) {
+    if (!started_ && queued_frames_ >= start_threshold_frames_) {
       io_task_runner_->PostTask(FROM_HERE, ready_for_playback_task_);
     }
   }