[WK2][EFL] Add requestCheckingOfString implementation.
https://bugs.webkit.org/show_bug.cgi?id=108172

Reviewed by Hajime Morrita.

Provide implementation of 'requestCheckingOfString' for WK2-EFL.
It uses the unified text checker feature.

* UIProcess/efl/TextCheckerEfl.cpp:
(WebKit::TextChecker::requestCheckingOfString):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144745 268f45cc-cd09-0410-ab3c-d52691b4dbfc