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/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 20797cc..e7a6cec 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -146,6 +146,10 @@
     "location.h",
     "mod_ops.h",
     "moving_max_counter.h",
+    "numerics/safe_compare.h",
+    "numerics/safe_conversions.h",
+    "numerics/safe_conversions_impl.h",
+    "numerics/safe_minmax.h",
     "onetimeevent.h",
     "pathutils.cc",
     "pathutils.h",
@@ -168,10 +172,6 @@
     "refcount.h",
     "refcountedobject.h",
     "refcounter.h",
-    "safe_compare.h",
-    "safe_conversions.h",
-    "safe_conversions_impl.h",
-    "safe_minmax.h",
     "sanitizer.h",
     "scoped_ref_ptr.h",
     "string_to_number.cc",
@@ -875,6 +875,8 @@
       "md5digest_unittest.cc",
       "mod_ops_unittest.cc",
       "moving_max_counter_unittest.cc",
+      "numerics/safe_compare_unittest.cc",
+      "numerics/safe_minmax_unittest.cc",
       "onetimeevent_unittest.cc",
       "pathutils_unittest.cc",
       "platform_thread_unittest.cc",
@@ -883,8 +885,6 @@
       "rate_statistics_unittest.cc",
       "ratetracker_unittest.cc",
       "refcountedobject_unittest.cc",
-      "safe_compare_unittest.cc",
-      "safe_minmax_unittest.cc",
       "string_to_number_unittest.cc",
       "stringencode_unittest.cc",
       "stringize_macros_unittest.cc",