Call lazyAttach before doing didNotifySubtreeInsertions

Script can run inside didNotifySubtreeInsertions so we should always lazyAttach
before that point so that when the script triggers style recalcs they will attach
the inserted nodes. HTMLFrameElementBase had a hack for this already but
what it was doing was not correct and was not general since other script that is
run in didNotifySubtreeInsertions needs the same behavior.

This also allows us to remove the ShouldSetAttached flag.

Review URL: https://chromiumcodereview.appspot.com/23983025

git-svn-id: svn://svn.chromium.org/blink/trunk@157608 bbb929c8-8fbe-4397-9dbb-9b2b20218538