Make RenderObject::adjustStyleDifference abort early for Text (and make the function more clear)

This function had a bunch of confusing cases in it, lets start by making
it clear that for Text we don't need to do most of the work. I also switched
to using <= instead of < for a bunch of cases to make it more obvious what
conditions this code is for.

BUG=335703

Review URL: https://codereview.chromium.org/148463012

git-svn-id: svn://svn.chromium.org/blink/trunk@165976 bbb929c8-8fbe-4397-9dbb-9b2b20218538