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}

webrtc/modules/audio_coding/codecs/g711/test/testG711.cc

@@ -86,6 +86,10 @@ int main(int argc, char* argv[]) {
   printf("G.711 version: %s\n\n", versionNumber);
   /* Get frame length */
   framelength = atoi(argv[1]);
+  if (framelength < 0) {
+    printf("  G.711: Invalid framelength %d.\n", framelength);
+    exit(1);
+  }
 
   /* Get compression law */
   strcpy(law, argv[2]);