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

media/base/buffers.cc

Index: media/base/buffers.cc
diff --git a/media/base/buffers.cc b/media/base/buffers.cc
index ab66b9adbd2156be59e3b83cc137ae1cf7d9e22a..582e4ca41f2b50be70ad89d9b78a35970842a1b9 100644
--- a/media/base/buffers.cc
+++ b/media/base/buffers.cc
@@ -9,6 +9,9 @@ namespace media {
 const base::TimeDelta kNoTimestamp =
     base::TimeDelta::FromMicroseconds(kint64min);
 
+const base::TimeDelta kInfiniteDuration =
+    base::TimeDelta::FromMicroseconds(kint64max);
+
 StreamSample::StreamSample() {}
 
 StreamSample::~StreamSample() {}