Intent to ship in 61: early blank first paint

[Florian Quèze]

Showing a blank window as early as possible during startup and then
filling it when the browser UI is ready improves perception of startup
performance. I implemented code to do this in bug 1336227. It's
controlled by the browser.startup.blankWindow preference, which I set
to true for Windows and Linux in bug 1447719 a month ago.
The regressions that have been filed since that are now fixed, and I
intend to let this ride the trains and ship it with 61.

If you've seen issues with this feature and haven't reported them yet,
please file a bug as soon as possible and make it block bug 1447719,
thanks!

-- 
Florian Quèze