Merge M51: "Fix audio glitch issue introduced by security fix for for…

…mat changes."

The patch was comparing the new sample rate value versus itself instead
of the old sample rate value versus the new.

BUG=602731
TEST=no more stuttering.

Review URL: https://codereview.chromium.org/1879143004

Cr-Commit-Position: refs/heads/master@{#386887}
(cherry picked from commit f4e5aba)

Review URL: https://codereview.chromium.org/1886173002 .

Cr-Commit-Position: refs/branch-heads/2704@{#46}
Cr-Branched-From: 6e53600-refs/heads/master@{#386251}

media/filters/ffmpeg_audio_decoder.cc

@@ -300,8 +300,10 @@ bool FFmpegAudioDecoder::FFmpegDecode(
       ChannelLayout channel_layout = ChannelLayoutToChromeChannelLayout(
           codec_context_->channel_layout, codec_context_->channels);
 
+      bool is_sample_rate_change =
+          av_frame_->sample_rate != config_.samples_per_second();
       bool is_config_stale =
-          av_frame_->sample_rate != config_.samples_per_second() ||
+          is_sample_rate_change ||
           channels != ChannelLayoutToChannelCount(config_.channel_layout()) ||
           av_frame_->format != av_sample_format_;
 
@@ -330,7 +332,7 @@ bool FFmpegAudioDecoder::FFmpegDecode(
                              config_.extra_data(), config_.encryption_scheme(),
                              config_.seek_preroll(), config_.codec_delay());
           config_changed = true;
-          if (av_frame_->sample_rate != config_.samples_per_second())
+          if (is_sample_rate_change)
             ResetTimestampState();
         } else {
           MEDIA_LOG(ERROR, media_log_)