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_processing/ns/nsx_core_neon.c

@@ -208,7 +208,7 @@ void WebRtcNsx_NoiseEstimationNeon(NoiseSuppressionFixedC* inst,
     uint16x8_t tmp16x8_4;
     int32x4_t tmp32x4;
 
-    for (i = 0; i < inst->magnLen - 7; i += 8) {
+    for (i = 0; i + 7 < inst->magnLen; i += 8) {
       // Compute delta.
       // Smaller step size during startup. This prevents from using
       // unrealistic values causing overflow.