Change Character::treatAs methods to use character names

Change the Character treatAsSpace, treatAsZeroWidthSpace and
treatAsZeroWidthSpaceInComplexScript methods to use character names
instead of character codes.

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/606243002

git-svn-id: svn://svn.chromium.org/blink/trunk@182817 bbb929c8-8fbe-4397-9dbb-9b2b20218538