RTC_[D]CHECK_op: Remove superfluous casts

There's no longer any need to make the two arguments have the same
signedness, so we can remove a bunch of superfluous (and sometimes
dangerous) casts.

It turned out I also had to fix the safe_cmp functions to properly handle
enums that are implicitly convertible to integers.

NOPRESUBMIT=true
BUG=webrtc:6645

Review-Url: https://codereview.webrtc.org/2534683002
Cr-Commit-Position: refs/heads/master@{#15281}
diff --git a/webrtc/modules/video_coding/video_codec_initializer.cc b/webrtc/modules/video_coding/video_codec_initializer.cc
index c6db916..dbdede0 100644
--- a/webrtc/modules/video_coding/video_codec_initializer.cc
+++ b/webrtc/modules/video_coding/video_codec_initializer.cc
@@ -164,7 +164,7 @@
   video_codec.minBitrate = streams[0].min_bitrate_bps / 1000;
   if (video_codec.minBitrate < kEncoderMinBitrateKbps)
     video_codec.minBitrate = kEncoderMinBitrateKbps;
-  RTC_DCHECK_LE(streams.size(), static_cast<size_t>(kMaxSimulcastStreams));
+  RTC_DCHECK_LE(streams.size(), kMaxSimulcastStreams);
   if (video_codec.codecType == kVideoCodecVP9) {
     // If the vector is empty, bitrates will be configured automatically.
     RTC_DCHECK(config.spatial_layers.empty() ||