Extract ElementTraversal.h from NodeTraversal.h

TEST=none
BUG=none
R=esprehn,eseidel,jamesr

Review URL: https://chromiumcodereview.appspot.com/22967007

git-svn-id: svn://svn.chromium.org/blink/trunk@156367 bbb929c8-8fbe-4397-9dbb-9b2b20218538