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

media/base/video_decoder_config.cc

Index: media/base/video_decoder_config.cc
diff --git a/media/base/video_decoder_config.cc b/media/base/video_decoder_config.cc
index 304dd61d66a0787d7364520c8948725abe747d51..ee334d3bd3eed24abfe8845719589c9f84928846 100644
--- a/media/base/video_decoder_config.cc
+++ b/media/base/video_decoder_config.cc
@@ -120,9 +120,9 @@ bool VideoDecoderConfig::IsValidConfig() const {
       frame_rate_denominator_ > 0 &&
       aspect_ratio_numerator_ > 0 &&
       aspect_ratio_denominator_ > 0 &&
-      natural_size_.width() <= Limits::kMaxDimension &&
-      natural_size_.height() <= Limits::kMaxDimension &&
-      natural_size_.GetArea() <= Limits::kMaxCanvas;
+      natural_size_.width() <= limits::kMaxDimension &&
+      natural_size_.height() <= limits::kMaxDimension &&
+      natural_size_.GetArea() <= limits::kMaxCanvas;
 }
 
 VideoCodec VideoDecoderConfig::codec() const {