[Chromecast] Fix unit test failures due to underflow.

chromecast/media/cma/backend/multizone_backend_unittest.cc

Index: chromecast/media/cma/backend/multizone_backend_unittest.cc
diff --git a/chromecast/media/cma/backend/multizone_backend_unittest.cc b/chromecast/media/cma/backend/multizone_backend_unittest.cc
index 7855798b87e6adb8df46ae92edbc21e63f64775f..08f03b421511c48e3b52e4ea9d72746a87b46e1e 100644
--- a/chromecast/media/cma/backend/multizone_backend_unittest.cc
+++ b/chromecast/media/cma/backend/multizone_backend_unittest.cc
@@ -268,7 +268,9 @@ void BufferFeeder::OnPushBufferComplete(BufferStatus status) {
     MediaPipelineBackend::AudioDecoder::RenderingDelay delay =
         decoder_->GetRenderingDelay();
 
-    if (delay.timestamp_microseconds != kNoTimestamp) {
+    if (delay.timestamp_microseconds == kNoTimestamp) {
+      next_push_playback_timestamp_ = kNoTimestamp;
+    } else {
       if (next_push_playback_timestamp_ == kNoTimestamp) {
         next_push_playback_timestamp_ =
             delay.timestamp_microseconds + delay.delay_microseconds;