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/merge.cc b/webrtc/modules/audio_coding/neteq/merge.cc
index fa033cf..23382ac 100644
--- a/webrtc/modules/audio_coding/neteq/merge.cc
+++ b/webrtc/modules/audio_coding/neteq/merge.cc
@@ -175,7 +175,7 @@
     // This is the truncated length.
   }
   // This assert should always be true thanks to the if statement above.
-  assert(210 * kMaxSampleRate / 8000 - *old_length >= 0);
+  assert(210 * kMaxSampleRate / 8000 >= *old_length);
 
   AudioMultiVector expanded_temp(num_channels_);
   expand_->Process(&expanded_temp);
@@ -342,7 +342,7 @@
   int start_index = timestamps_per_call_ +
       static_cast<int>(expand_->overlap_length());
   start_index = std::max(start_position, start_index);
-  start_index = std::max(start_index - input_length, 0);
+  start_index = (input_length > start_index) ? 0 : (start_index - input_length);
   // Downscale starting index to 4kHz domain. (fs_mult_ * 2 = fs_hz_ / 4000.)
   int start_index_downsamp = start_index / (fs_mult_ * 2);