Update SourceBufferStream and its unit tests to always expect valid durations.

media/base/decoder_buffer.h

Index: media/base/decoder_buffer.h
diff --git a/media/base/decoder_buffer.h b/media/base/decoder_buffer.h
index 27de88f4692e3d408d81508bc913c561c2712dc7..c17aa213ab121c24a0abec1214a7e7d9a55ae8f4 100644
--- a/media/base/decoder_buffer.h
+++ b/media/base/decoder_buffer.h
@@ -14,6 +14,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
+#include "media/base/buffers.h"
 #include "media/base/decrypt_config.h"
 #include "media/base/media_export.h"
 
@@ -77,6 +78,9 @@ class MEDIA_EXPORT DecoderBuffer
 
   void set_duration(base::TimeDelta duration) {
     DCHECK(!end_of_stream());
+    DCHECK(duration == kNoTimestamp() ||
+           (duration >= base::TimeDelta() && duration != kInfiniteDuration()))
+        << duration.InSecondsF();
     duration_ = duration;
   }