[Chromecast] Add support for different playback rates to ALSA backend

chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc

Index: chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
diff --git a/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc b/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
index a98d1f73740ed0d7f45659b345c06723936c1618..d75b80add5d43492b252bdf31ace88405b5fac3a 100644
--- a/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
+++ b/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
@@ -81,9 +81,9 @@ bool MediaPipelineBackendAlsa::Resume() {
 }
 
 bool MediaPipelineBackendAlsa::SetPlaybackRate(float rate) {
-  // TODO(kmackay) Implement this for rates other than 1.0.
-  if (rate != 1.0)
-    NOTIMPLEMENTED() << " unhandled rate: " << rate;
+  if (audio_decoder_) {
+    return audio_decoder_->SetPlaybackRate(rate);
+  }
   return true;
 }