Clicking print/save on about:preferences#payments causes a crash on cr54

[luixxiul]

Describe the issue you encountered: Clicking print/save on about:preferences#payments causes a crash on chromium54

An uncaught exception occurred in the main process Uncaught Exception:
TypeError: Cannot read property 'then' of undefined
    at fs.writeFile (/Users/Suguru/browser-laptop/app/ledger.js:269:44)
    at FSReqWrap.oncomplete (fs.js:111:15)
/Users/Suguru/browser-laptop/node_modules/node-anonize2-relic-emscripten/anonize2.js:18
if(!Module)Module=(typeof Module!=="undefined"?Module:null)||{};var moduleOverrides={};for(var key in Module){if(Module.hasOwnProperty(key)){moduleOverrides[key]=Module[key]}}var ENVIRONMENT_IS_WEB=false;var ENVIRONMENT_IS_WORKER=false;var ENVIRONMENT_IS_NODE=false;var ENVIRONMENT_IS_SHELL=false;if(Module["ENVIRONMENT"]){if(Module["ENVIRONMENT"]==="WEB"){ENVIRONMENT_IS_WEB=true}else if(Module["ENVIRONMENT"]==="WORKER"){ENVIRONMENT_IS_WORKER=true}else if(Module["ENVIRONMENT"]==="NODE"){ENVIRONMENT_IS_NODE=true}else if(Module["ENVIRONMENT"]==="SHELL"){ENVIRONMENT_IS_SHELL=true}else{throw new Error("The provided Module['ENVIRONMENT'] value is not valid. It must be one of: WEB|WORKER|NODE|SHELL.")}}else{ENVIRONMENT_IS_WEB=typeof window==="object";ENVIRONMENT_IS_WORKER=typeof importScripts==="function";ENVIRONMENT_IS_NODE=typeof process==="object"&&typeof require==="function"&&!ENVIRONMENT_IS_WEB&&!ENVIRONMENT_IS_WORKER;ENVIR

TypeError: Cannot read property 'then' of undefined
    at fs.writeFile (/Users/Suguru/browser-laptop/app/ledger.js:269:44)
    at FSReqWrap.oncomplete (fs.js:111:15)
process exited with code 7

Expected behavior: It should open the new tab with the dialog for print/save the key file

• Platform (Win7, 8, 10? macOS? Linux distro?): macOS

• Brave Version: b824ae6

• Steps to reproduce:

  1. Open about:preferences#payments
  2. Click "Advanced Settings..."
  3. Click "Backup your wallet"
  4. Click print/save

• Any related issues:

CC @mrose17

[bbondy]

I'll fix

[bbondy]

It was crashing because it was treating this api as a promise but really it uses callbacks.

Note that the print operation won't actually work until #6159 is addressed.

[luixxiul]

Test plan:
Open about:preferences#payments
Click "Advanced Settings..."
Click "Backup your wallet"
Click print/save