Make OS audio buffer size limits visible.

media/base/limits.h

Index: media/base/limits.h
diff --git a/media/base/limits.h b/media/base/limits.h
index f936c8bf9257a064c9aeac0229bec51a65ffcb15..fb0cf435c5d9a5da4be23bdbff746c0a662c1aab 100644
--- a/media/base/limits.h
+++ b/media/base/limits.h
@@ -7,6 +7,8 @@
 #ifndef MEDIA_BASE_LIMITS_H_
 #define MEDIA_BASE_LIMITS_H_
 
+#include "build/build_config.h"
+
 namespace media {
 
 namespace limits {
@@ -58,6 +60,18 @@ enum {
   kMaxInitDataLength = 64 * 1024,         // 64 KB
   kMaxSessionResponseLength = 64 * 1024,  // 64 KB
   kMaxKeySystemLength = 256,
+
+// Minimum and maximum buffer sizes for certain audio platforms.
+#if defined(OS_MACOSX)
+  kMinAudioBufferSize = 128,
+  kMaxAudioBufferSize = 4096,

[Raymond Toy, 2017/06/27 16:06:48]

Any particular reason why 4096 and not 8192 like you have for the other
platforms?

A comment on how you picked 8192 would be good too.

[Andrew MacPherson, 2017/06/27 17:03:40]

4096 was just copied from audio_manager_mac.cc, I've added a comment just saying
it was taken from there. I'm not sure what the underlying reason for 4096 or
8192 is, these were just the values that were already being used by the
different AudioManagers.

+#elif defined(USE_PULSEAUDIO)
+  kMinAudioBufferSize = 512,

[Raymond Toy, 2017/06/27 16:06:48]

A comment here explaining why the min is 512 would be good.  Something as simple
as saying that experiments with smaller size frequently produces glitches.

[Andrew MacPherson, 2017/06/27 17:03:41]

Same as above, this is the value that was already in use and I'm not sure what
the original reasoning behind it was. I can definitely add a comment if you
think that's best but I can't confirm the reasons for these buffer sizes.

+  kMaxAudioBufferSize = 8192,
+#elif defined(USE_CRAS)
+  kMinAudioBufferSize = 256,

[Raymond Toy, 2017/06/27 16:06:48]

Likewise a comment on the min value would be good.

[Andrew MacPherson, 2017/06/27 17:03:41]

I've added a short comment here, does that work?

+  kMaxAudioBufferSize = 8192,
+#endif
 };
 
 }  // namespace limits