tabs completer: do not use tab.favIconUrl
tabs completer: do not use tab.favIconUrl

so that Chrome can reuse cached file, whatever the cache policy is
* even if network has been turned off
* even if the icon file has "no-cache"
* even if "Disable cache" in Developer Tools is checked
* in case the favicon url is collecting client information

it also does good to filter out network requests

fix the bug that some tabs have no favIconUrl (e.g. chrome://bookmarks)

fix the crash that some favIconUrl is not allowed to load:
    e.g. chrome://extensions

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus