Merge HTMLBodyElement::didNotifySubtreeInsertions into HTMLBodyElement::insertedInto

The logic in didNotifySubtreeInsertions to set the marginwidth and marginheight attributes
on the <body> of elements inside <iframe> and <frame> doesn't need to run after inserting
all the children of the frame. In fact this means that when you have those attributes
and then the script in the iframe touches offsetLeft or any layout dependent property
we'll layout with the wrong values and then have to do another layout after these margin
attributes are set.

I also remove the scheduleRelayout() call that was inside didNotifySubtreeInsertions. This
call doesn't make any sense, inserting a <body> will always trigger a style recalc and
a subsequent layout. The code is 9 years old: https://trac.webkit.org/changeset/8122
and all tests run fine without it.

Review URL: https://chromiumcodereview.appspot.com/23500002

git-svn-id: svn://svn.chromium.org/blink/trunk@156756 bbb929c8-8fbe-4397-9dbb-9b2b20218538