Disables filepicker on old Chrome versions

Displays an error dialog if the user tries to mount a local directory, when the version of Chrome does not support this operation.
maybe make this a function, because i imagine there will be more features and more version controls..
this code we should write only once. 
if  (colab.checkVersionAndWarnUser(colab.MOUNT_LOCAL_DIRECTORY_MIN_CHROME_VERSION)) {
  colab.app.postMessage(...)
}