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

media/base/buffers.h

Index: media/base/buffers.h
diff --git a/media/base/buffers.h b/media/base/buffers.h
index 82f240290512ae66b028dc9fdfa38533f01dffa5..400eaf7a6229d4d64025eba72d7d71806427ec97 100644
--- a/media/base/buffers.h
+++ b/media/base/buffers.h
@@ -37,6 +37,9 @@ namespace media {
 // Indicates an invalid or missing timestamp.
 MEDIA_EXPORT extern const base::TimeDelta kNoTimestamp;
 
+// Represents an infinite stream duration.
+MEDIA_EXPORT extern const base::TimeDelta kInfiniteDuration;
+
 class MEDIA_EXPORT StreamSample
     : public base::RefCountedThreadSafe<StreamSample> {
  public: