[8u post commit review]: RE-32560: Changing user_agent stylesheet on scene or subscene

[David Grieve]

Jonathan,

Could  you please review:
   http://hg.openjdk.java.net/openjfx/8u-dev/rt/rev/fe6fe6108a11


for https://javafx-jira.kenai.com/browse/RT-35260. I still need to 
update the cssref and to add unit tests.