Replace media::Limits struct with media::limits namespace and update documentation.

media/base/audio_decoder_config.cc

Index: media/base/audio_decoder_config.cc
diff --git a/media/base/audio_decoder_config.cc b/media/base/audio_decoder_config.cc
index 243c4c5b8c310041a11daae9c2326dac653dec85..ece264b8b38eb455d31521b31b99fe1e180381b5 100644
--- a/media/base/audio_decoder_config.cc
+++ b/media/base/audio_decoder_config.cc
@@ -100,9 +100,9 @@ bool AudioDecoderConfig::IsValidConfig() const {
   return codec_ != kUnknownAudioCodec &&
       channel_layout_ != CHANNEL_LAYOUT_UNSUPPORTED &&
       bits_per_channel_ > 0 &&
-      bits_per_channel_ <= Limits::kMaxBitsPerSample &&
+      bits_per_channel_ <= limits::kMaxBitsPerSample &&
       samples_per_second_ > 0 &&
-      samples_per_second_ <= Limits::kMaxSampleRate;
+      samples_per_second_ <= limits::kMaxSampleRate;
 }
 
 AudioCodec AudioDecoderConfig::codec() const {