Allow more zooming

Set maximum-scale to the default for Safari (5.0):
https://developer.apple.com/library/ios/documentation/appleapplications/reference/safariwebcontent/usingtheviewport/usingtheviewport.html

Also, remove a FIXME that its author doesn't understand.

Bug: 66892
Change-Id: I8b9c76e5f18c07dbbc7a376b4bfdabafe51a9ffd