/usr/share/javascript/prettify/lang-proto.js is in libjs-prettify 2013.03.04+dfsg-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | PR.registerLangHandler(PR.sourceDecorator({keywords:"bytes,default,double,enum,extend,extensions,false,group,import,max,message,option,optional,package,repeated,required,returns,rpc,service,syntax,to,true",types:/^(bool|(double|s?fixed|[su]?int)(32|64)|float|string)\b/,cStyleComments:!0}),["proto"]);
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