Unblacklist Opera >= 12

Opera 12 seems to work well enough, but I'm not confident enough to
whitelist it just yet.

Opera 15 is basically Chrome with a different interface, so it should
work perfectly, but it's barely out of beta and untested right now.

Bug: 36000
Change-Id: Ia80a6f53f8c128ef52d0bfde1828fdc132046afb