Add transition for new-tabbackground to avoid flash of unstyled content
Add transition for new-tabbackground
to avoid flash of unstyled content

Auditors: @bsclifton

Closes #5309

Test Plan:
* Enable new tab page
* Open a new tab
* There should be a smooth transition between the moment page is opened and background is charged

willy-b/browser-laptop,jonathansampson/browser-laptop,darkdh/browser-laptop,darkdh/browser-laptop,timborden/browser-laptop,willy-b/browser-laptop,jonathansampson/browser-laptop,willy-b/browser-laptop,luixxiul/browser-laptop,timborden/browser-laptop,luixxiul/browser-laptop,willy-b/browser-laptop,jonathansampson/browser-laptop,diasdavid/browser-laptop,pmkary/braver,diasdavid/browser-laptop,dcposch/browser-laptop,dcposch/browser-laptop,Sh1d0w/browser-laptop,timborden/browser-laptop,darkdh/browser-laptop,dcposch/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,darkdh/browser-laptop,pmkary/braver,pmkary/braver,dcposch/browser-laptop,luixxiul/browser-laptop,Sh1d0w/browser-laptop,pmkary/braver,diasdavid/browser-laptop,Sh1d0w/browser-laptop,timborden/browser-laptop,Sh1d0w/browser-laptop,jonathansampson/browser-laptop