Prepare to convert various types to size_t.

This makes some behaviorally-invariant changes to make certain code that
currently only works correctly with signed types work safely regardless of the
signedness of the types in question.  This is preparation for a future change
that will convert a variety of types to size_t.

There are also some formatting changes (e.g. converting "enum hack" usage to real consts) to make it simpler to just change "int" to "size_t" in the future to change the types of those constants.

BUG=none
R=andrew@webrtc.org, juberti@webrtc.org, kwiberg@webrtc.org
TBR=ajm

Review URL: https://codereview.webrtc.org/1174813003

Cr-Commit-Position: refs/heads/master@{#9413}
diff --git a/webrtc/modules/audio_coding/neteq/test/RTPencode.cc b/webrtc/modules/audio_coding/neteq/test/RTPencode.cc
index 7d8d60d..1ef9ce5 100644
--- a/webrtc/modules/audio_coding/neteq/test/RTPencode.cc
+++ b/webrtc/modules/audio_coding/neteq/test/RTPencode.cc
@@ -450,7 +450,10 @@
   CHECK_NOT_NULL(out_file);
   printf("Output file: %s\n\n", argv[2]);
   packet_size = atoi(argv[3]);
-  CHECK_NOT_NULL(packet_size);
+  if (packet_size <= 0) {
+     printf("Packet size %d must be positive", packet_size);
+     return -1;
+  }
   printf("Packet size: %i\n", packet_size);
 
   // check for stereo