Revert "GN rtc_* templates: Set default visibility to webrtc_root + "/*""

This reverts commit 588c548657b3ddf76e7b3f241263eef7f5799f16.

Reason for revert: 

Breaks Chrome FYI:

/b/c/b/Linux_Builder/src/buildtools/linux64/gn gen //out/Release --check
  -> returned 1
ERROR at //build/split_static_library.gni:12:5: Dependency not allowed.
    static_library(target_name) {
    ^----------------------------
The item //content/renderer:renderer
can not depend on //third_party/webrtc/media:rtc_internal_video_codecs
because it is not in //third_party/webrtc/media:rtc_internal_video_codecs's visibility list: [
  //third_party/webrtc/*
  //third_party/webrtc_overrides/*
]

 https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.webrtc.fyi%2FLinux_Builder%2F23560%2F%2B%2Frecipes%2Fsteps%2Fgenerate_build_files%2F0%2Fstdout

Original change's description:
> GN rtc_* templates: Set default visibility to webrtc_root + "/*"
> 
> This means that by default, targets are visible to everything under
> the WebRTC root, but not visible to anything else.
> 
> API targets are manually tagged with visibility "*", so that targets
> outside the WebRTC tree can see them.
> 
> BUG=webrtc:8254
> 
> Change-Id: Icdbee6e0d22d93240ff2fb530c8f9dc48e351509
> Reviewed-on: https://webrtc-review.googlesource.com/24140
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21548}

TBR=mbonadei@webrtc.org,kwiberg@webrtc.org

Change-Id: I06620ce3d6f67482935c22efa231dd6cab91625a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8254
Reviewed-on: https://webrtc-review.googlesource.com/38760
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21555}

stats/BUILD.gn

@@ -15,7 +15,6 @@ group("stats") {
 }
 
 rtc_static_library("rtc_stats") {
-  visibility = [ "*" ]
   cflags = []
   sources = [
     "rtcstats.cc",
@@ -30,7 +29,6 @@ rtc_static_library("rtc_stats") {
 }
 
 rtc_source_set("rtc_stats_test_utils") {
-  visibility = [ "*" ]
   cflags = []
   sources = [
     "test/rtcteststats.cc",