Fix some sign-compare warnings in webrtc/api.

The disabling of the warnings doesn't seem to work when Chromium
is using our targets (https://codereview.chromium.org/2022833002)
so better fix them.

BUG=webrtc:4256,webrtc:3307
NOTRY=True

Review-Url: https://codereview.webrtc.org/2074423002
Cr-Commit-Position: refs/heads/master@{#13217}
diff --git a/webrtc/api/webrtcsdp.cc b/webrtc/api/webrtcsdp.cc
index b2e75a2..0b8d6f6 100644
--- a/webrtc/api/webrtcsdp.cc
+++ b/webrtc/api/webrtcsdp.cc
@@ -2190,7 +2190,8 @@
     int payload_type = *it;
     if (!media_desc->HasCodec(payload_type) &&
         payload_type >= 0 &&
-        payload_type < arraysize(kStaticPayloadAudioCodecs)) {
+        static_cast<uint32_t>(payload_type) <
+            arraysize(kStaticPayloadAudioCodecs)) {
       std::string encoding_name = kStaticPayloadAudioCodecs[payload_type].name;
       int clock_rate = kStaticPayloadAudioCodecs[payload_type].clockrate;
       size_t channels = kStaticPayloadAudioCodecs[payload_type].channels;