Move presentation attribute style code out of Element.cpp

This code doesn't need to be in Element.cpp, that file is way
too big and this is all self contained.

Review URL: https://codereview.chromium.org/27516012

git-svn-id: svn://svn.chromium.org/blink/trunk@159932 bbb929c8-8fbe-4397-9dbb-9b2b20218538