Chrome ToT (15bab7ea, r500110), sometime clicking on "Add Person" result in a blank page. The reason is that the webview.contentWindow is null when 'contentload' event is fired. There seems to be a race of when the event fires and when the contentWindow property is set.

It is not easy to repro but it does happen.

Example js call stack:
login.js:9960 TypeError: Cannot read property 'postMessage' of null
    at Function.PostMessageChannel.init (chrome://oobe/login.js:16283:26)
    at SamlHandler.onContentLoad_ (chrome://oobe/login.js:17255:26)
    at WebViewImpl.GuestViewContainer.dispatchEvent (extensions::guestViewContainer:183:23)
    at WebViewEvents.<anonymous> (extensions::guestViewEvents:129:15)
    at extensions::guestViewEvents:189:22
    at EventImpl.dispatchToListener (extensions::event_bindings:403:22)
    at Event.publicClassPrototype.(anonymous function) [as dispatchToListener] (extensions::utils:138:26)
    at EventImpl.dispatch_ (extensions::event_bindings:387:35)
    at dispatchArgs (extensions::event_bindings:260:26)
    at dispatchEvent (extensions::event_bindings:269:7)