Remove ShouldSetAttached flag to lazyAttach

This flag does not seem needed, there's no apparent reason that frames
would want to do setNeedsStyleRecalc(LazyAttachStyleChange) but not
setAttached().

Review URL: https://chromiumcodereview.appspot.com/24066002

git-svn-id: svn://svn.chromium.org/blink/trunk@157423 bbb929c8-8fbe-4397-9dbb-9b2b20218538