Make tests related to undo word deletion platform independent

This patch makes the following tests platform independent.
editing/undo/undo-combined-delete-boundary.html
editing/undo/undo-combined-delete.html
editing/undo/undo-delete-boundary.html
editing/undo/undo-delete.html
editing/undo/undo-forward-delete-boundary.html
editing/undo/undo-forward-delete.html

R=ojan, yosin, leviw

Review URL: https://codereview.chromium.org/43523002

git-svn-id: svn://svn.chromium.org/blink/trunk@160755 bbb929c8-8fbe-4397-9dbb-9b2b20218538