Reland "Upconvert various types to int.", neteq portion.

This reverts portions of commit cb180976dd0e9672cde4523d87b5f4857478b5e9, which
reverted commit 83ad33a8aed1fb00e422b6abd33c3e8942821c24.  Specifically, the
files in webrtc/modules/audio_coding/neteq/ are relanded.

The original commit message is below:

Upconvert various types to int.

Per comments from HL/kwiberg on https://webrtc-codereview.appspot.com/42569004 , when there is existing usage of mixed types (int16_t, int, etc.), we'd prefer to standardize on larger types like int and phase out use of int16_t.

Specifically, "Using int16 just because we're sure all reasonable values will fit in 16 bits isn't usually meaningful in C."

This converts some existing uses of int16_t (and, in a few cases, other types such as uint16_t) to int (or, in a few places, int32_t).  Other locations will be converted to size_t in a separate change.

BUG=none
TBR=kwiberg

Review URL: https://codereview.webrtc.org/1181073002

Cr-Commit-Position: refs/heads/master@{#9427}

webrtc/modules/audio_coding/neteq/neteq_impl.cc

@@ -1278,7 +1278,7 @@ int NetEqImpl::DecodeLoop(PacketList* packet_list, Operations* operation,
            *operation == kPreemptiveExpand);
     packet_list->pop_front();
     size_t payload_length = packet->payload_length;
-    int16_t decode_length;
+    int decode_length;
     if (packet->sync_packet) {
       // Decode to silence with the same frame size as the last decode.
       LOG(LS_VERBOSE) << "Decoding sync-packet: " <<