WIP private browsing mode windows work re #1

Nits:

* pb mode window doesn't open a tab with the 'pb mode explaination text'
in it.
* re-open a p-b mode window doesn't activate it to the front.
* it takes a long time for the newly opened window to get the 'mask'
chrome annotation.
Merge pull request #3 from gregglind/privatewindow

WIP private browsing mode windows work re #1