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 d6a50d40c5ca3d65df32209136da1181bc20d7db..9ee736842790afd31742b971cdd5544df5194477 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 {
@@ -54,6 +56,28 @@ enum {
   kMaxInitDataLength = 64 * 1024,         // 64 KB
   kMaxSessionResponseLength = 64 * 1024,  // 64 KB
   kMaxKeySystemLength = 256,
+
+  // Minimum and maximum buffer sizes per audio platform.
+#if defined(OS_MACOSX)
+  kMinimumInputOutputBufferSize = 128,

[o1ka, 2017/06/09 10:52:39]

To keep it simple, maybe just name them kMinAudioBufferSize and
kMaxAudioBufferSize on all the platforms?

[Andrew MacPherson, 2017/06/12 11:23:33]

Done.

+  kMaximumInputOutputBufferSize = 4096,
+  kMinimumOutputBufferSize = kMinimumInputOutputBufferSize,
+  kMaximumOutputBufferSize = kMaximumInputOutputBufferSize,
+#elif defined(USE_PULSEAUDIO)
+  kMinimumOutputBufferSize = 512,
+  kMaximumOutputBufferSize = 8192,
+#elif defined(USE_CRAS)
+  kMinimumOutputBufferSize = 512,

[o1ka, 2017/06/09 10:52:39]

This is a bit confusing, see
AudioManagerCras::GetMinimumOutputBufferSizePerBoard.
If you want to drop to 256 on CRAS it would probably be easier to do it in this
CL.
Or if we are going to have various platform-specific calculations for min/max,
then it may be easier to keep minx/max in AudioLatency along with those
calculations.

[Andrew MacPherson, 2017/06/12 11:23:33]

Done, I've dropped the minimum to 256 now. Maybe
GetMinimumOutputBufferSizePerBoard should be renamed to
GetPreferredOutputBufferSizePerBoard?

I don't currently have a Chromium for ChromeOS development environment set up so
I haven't verified that this change behaves as expected there. I will add unit
tests as you suggest above though to ensure that this case is tested. 

+  kMaximumOutputBufferSize = 8192,
+#elif defined(OS_WIN)
+  // TODO(andrew.macpherson): I can't see any maximum buffer size specified for
+  // either Android or Windows, so I'm not sure if this should be INT_MAX or
+  // something else. Specifying 4096 so that there is no behaviour change since
+  // we had hardcoded 4096 as the maximum previously.
+  kMaximumOutputBufferSize = 4096,
+#elif defined(OS_ANDROID)
+  kMaximumOutputBufferSize = 4096,
+#endif
 };
 
 }  // namespace limits