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

Specifically, I'm moving

  histogram_percentile_counter.h
  mathutils.h
  mod_ops.h
  moving_max_counter.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: I78a999984a27ef935be2d7c3136475d5f209adda
Reviewed-on: https://webrtc-review.googlesource.com/20870
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20832}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index e7a6cec..873a3eb 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -139,13 +139,13 @@
     "flags.h",
     "format_macros.h",
     "function_view.h",
-    "histogram_percentile_counter.cc",
-    "histogram_percentile_counter.h",
     "ignore_wundef.h",
     "location.cc",
     "location.h",
-    "mod_ops.h",
-    "moving_max_counter.h",
+    "numerics/histogram_percentile_counter.cc",
+    "numerics/histogram_percentile_counter.h",
+    "numerics/mod_ops.h",
+    "numerics/moving_max_counter.h",
     "numerics/safe_compare.h",
     "numerics/safe_conversions.h",
     "numerics/safe_conversions_impl.h",
@@ -610,7 +610,7 @@
       "callback.h",
       "logsinks.cc",
       "logsinks.h",
-      "mathutils.h",
+      "numerics/mathutils.h",
       "optionsfile.cc",
       "optionsfile.h",
       "rollingaccumulator.h",
@@ -870,11 +870,11 @@
       "event_unittest.cc",
       "file_unittest.cc",
       "function_view_unittest.cc",
-      "histogram_percentile_counter_unittest.cc",
       "logging_unittest.cc",
       "md5digest_unittest.cc",
-      "mod_ops_unittest.cc",
-      "moving_max_counter_unittest.cc",
+      "numerics/histogram_percentile_counter_unittest.cc",
+      "numerics/mod_ops_unittest.cc",
+      "numerics/moving_max_counter_unittest.cc",
       "numerics/safe_compare_unittest.cc",
       "numerics/safe_minmax_unittest.cc",
       "onetimeevent_unittest.cc",