[Chromecast] Fix unit test failures due to underflow.

chromecast/media/cma/backend/multizone_backend_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdint.h>
 #include <stdlib.h>
 
 #include <algorithm>
 #include <limits>
 #include <memory>
@@ skipping 250 matching lines @@
 
 void BufferFeeder::OnPushBufferComplete(BufferStatus status) {
   DCHECK(thread_checker_.CalledOnValidThread());
   pending_buffer_ = nullptr;
 
   if (!effects_only_) {
     ASSERT_NE(status, MediaPipelineBackend::kBufferFailed);
     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;
       } else {
         int64_t expected_next_push_playback_timestamp =
             next_push_playback_timestamp_ + last_push_length_us_;
         next_push_playback_timestamp_ =
             delay.timestamp_microseconds + delay.delay_microseconds;
         int64_t error = next_push_playback_timestamp_ -
                         expected_next_push_playback_timestamp;
@@ skipping 109 matching lines @@
                      ::testing::Values(0.5f, 0.99f, 1.0f, 1.01f, 2.0f)));
 
 INSTANTIATE_TEST_CASE_P(
     Optional,
     MultizoneBackendTest,
     testing::Combine(::testing::Values(64000, 88200, 96000),
                      ::testing::Values(0.5f, 0.99f, 1.0f, 1.01f, 2.0f)));
 
 }  // namespace media
 }  // namespace chromecast