Link Japanese translation

See triple-underscore/triple-underscore.github.io#1 (comment).

Author: annevk

fullscreen.bs

@@ -4,6 +4,7 @@ H1: Fullscreen API
 Shortname: fullscreen
 Text Macro: TWITTER fullscreenapi
 Abstract: The Fullscreen API standard defines an API for elements to display themselves fullscreen.
+Translation: ja https://triple-underscore.github.io/fullscreen-ja.html
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 <pre class=link-defaults>
@@ -672,6 +673,7 @@ Simon Pieters,
 Tab Atkins,
 Takayoshi Kochi,
 Theresa O'Connor,
+triple-underscore,
 Vincent Scheib, and
 Xidorn Quan
 for also being awesome.