Move some numeric utility code from rtc_base/ to rtc_base/numerics/

Specifically, I'm moving

  safe_compare.h
  safe_conversions.h
  safe_minmax.h

They shouldn't be part of the API, and moving them to an appropriate
subdirectory of rtc_base/ is a good way to keep track of that.

BUG=webrtc:8445

Change-Id: I458531aeb30bcf4291c4bec3bf22a2fffbf054ff
Reviewed-on: https://webrtc-review.googlesource.com/20860
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20829}
diff --git a/modules/audio_coding/codecs/cng/audio_encoder_cng_unittest.cc b/modules/audio_coding/codecs/cng/audio_encoder_cng_unittest.cc
index 303529e..6e246c9 100644
--- a/modules/audio_coding/codecs/cng/audio_encoder_cng_unittest.cc
+++ b/modules/audio_coding/codecs/cng/audio_encoder_cng_unittest.cc
@@ -14,7 +14,7 @@
 #include "common_audio/vad/mock/mock_vad.h"
 #include "modules/audio_coding/codecs/cng/audio_encoder_cng.h"
 #include "rtc_base/constructormagic.h"
-#include "rtc_base/safe_conversions.h"
+#include "rtc_base/numerics/safe_conversions.h"
 #include "test/gtest.h"
 #include "test/mock_audio_encoder.h"
 
diff --git a/modules/audio_coding/codecs/cng/webrtc_cng.cc b/modules/audio_coding/codecs/cng/webrtc_cng.cc
index e2a3347..8b8e57e 100644
--- a/modules/audio_coding/codecs/cng/webrtc_cng.cc
+++ b/modules/audio_coding/codecs/cng/webrtc_cng.cc
@@ -13,7 +13,7 @@
 #include <algorithm>
 
 #include "common_audio/signal_processing/include/signal_processing_library.h"
-#include "rtc_base/safe_conversions.h"
+#include "rtc_base/numerics/safe_conversions.h"
 
 namespace webrtc {