Disable UIComponent for XML documents
Disable UIComponent for XML documents

If the page is an XML document, nothing we do works:
* <style> elements show their contents inline,
* <iframe> elements don't load any content,
* document.createElement generates elements that
  - have Element.style == null, and
  - ignore CSS.

This commit stops us from injecting anything into the DOM from
UIComponent, fixing #1640.

douglas-larocca/vimium,borgified/vimium,willsALMANJ/vimium,Lw-Cui/vimium,elegantwww/vimium,tuchangwei/vimium,VPashkov/vimium,hhsue/vimium,elderbas/vimium,philc/vimium,VPashkov/vimium,poacher2k/vimium,PrestanceDesign/vimium,cyrixhero/vimium,douglas-larocca/vimium,wildeyes/vimium,bkudria/vimium,dimatter/vimium,mrmr1993/vimium,elegantwww/vimium,decaffeinate-examples/vimium,elderbas/vimium,wildeyes/vimium,willsALMANJ/vimium,csmalin/vimium,borgified/vimium,zhangmin510/vimium,justalittlenoob/vimium,zhangmin510/vimium,PrestanceDesign/vimium,hhsue/vimium,Aleroniponi/vimium,PickRelated/vimium,zhangmin510/vimium,borgified/vimium,philc/vimium,gdh1995/vimium,decaffeinate-examples/vimium,decaffeinate-examples/vimium,Lw-Cui/vimium,Aleroniponi/vimium,elegantwww/vimium,csmalin/vimium,cyrixhero/vimium,justalittlenoob/vimium,Aleroniponi/vimium,Robinson10240/vimium,cyrixhero/vimium,mrmr1993/vimium,gdh1995/vimium,hhsue/vimium,Lw-Cui/vimium,Robinson10240/vimium,wildeyes/vimium,poacher2k/vimium,mrmr1993/vimium,tuchangwei/vimium,bkudria/vimium,smblott-github/vimium,gdh1995/vimium,smblott-github/vimium,bkudria/vimium,Robinson10240/vimium,smblott-github/vimium,dimatter/vimium,philc/vimium,tuchangwei/vimium,dimatter/vimium,VPashkov/vimium,PickRelated/vimium,poacher2k/vimium,PrestanceDesign/vimium,PickRelated/vimium,willsALMANJ/vimium,csmalin/vimium,elderbas/vimium,douglas-larocca/vimium,justalittlenoob/vimium