https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/391482

Click the Show/Hide Gantt Chart button gives this error in the JS console:

Uncaught TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
    at reconcileSwarmingSteps (gantt_chart.js:29)
    at toggleChart (gantt_chart.js:82)
    at HTMLButtonElement.onclick (391482:5009)

rawSteps is undefined because it's declared like this:

  var rawSteps = [
          {'name': 'steps',
       'start': new Date(1487223452.85*1000),
       'end': new Date (None*1000)},
          {'name': 'update_scripts',
  ...

Uncaught ReferenceError: None is not defined
    at 391482:137