Add Wall of Browser Bugs entries for Selectors Level 4 :nth-child() (#20143)

Refs http://caniuse.com/#feat=css-nth-child-of
Refs https://drafts.csswg.org/selectors-4/#the-nth-child-pseudo
Refs https://crbug.com/304163
Refs https://bugzil.la/854148

[skip sauce]

docs/_data/browser-bugs.yml

@@ -148,6 +148,16 @@
   origin: >
     Bootstrap#19670
 
+-
+  browser: >
+    Firefox
+  summary: >
+    Implement the `of <selector-list>` clause of the `:nth-child()` pseudo-class
+  upstream_bug: >
+    Mozilla#854148
+  origin: >
+    Bootstrap#20143
+
 -
   browser: >
     Firefox (Windows)
@@ -248,6 +258,16 @@
   origin: >
     Bootstrap#12832
 
+-
+  browser: >
+    Chrome
+  summary: >
+    Implement the `of <selector-list>` clause of the `:nth-child()` pseudo-class
+  upstream_bug: >
+    Chromium#304163
+  origin: >
+    Bootstrap#20143
+
 -
   browser: >
     Chrome (Windows & Linux)