Make LayoutTests/editing/selection/extend-by-word-002.html platform independent

This patch makes LayoutTests/editing/selection/extend-by-word-002.html platform dependent by using js-test-pre framework rather than render tree dump with pixel dump. 

This patch is preparation of fixing issue 174349 "	Contenteditable with :first-letter styling".
 
BUG=n/a
TEST=n/a

Review URL: https://codereview.chromium.org/26236002

git-svn-id: svn://svn.chromium.org/blink/trunk@159078 bbb929c8-8fbe-4397-9dbb-9b2b20218538