Change *webtai* scheme with *wtai*

*webtai* scheme isn't correct scheme. *wtai* is correct. Now, latest custom-handler spec
change it with *wtai*.

Spec: http://www.whatwg.org/specs/web-apps/current-work/#whitelisted-scheme
See also: http://en.wikipedia.org/wiki/URI_scheme

BUG=

Review URL: https://chromiumcodereview.appspot.com/22875010

git-svn-id: svn://svn.chromium.org/blink/trunk@155998 bbb929c8-8fbe-4397-9dbb-9b2b20218538