when using tools/json_schema_compiler/compiler.py -g externs one can get both relative and absolute paths in the "This file was generated by" message.

Ex: https://codereview.chromium.org/2922673003/diff/1/third_party/closure_compiler/externs/automation.js?context=10&column_width=80&tab_spaces=8

because absolute, local paths on my box probably wont work for others (and will always change), we probably shouldn't do this ;)