[GTK] Migrate WebKitWebResource to GTask
https://bugs.webkit.org/show_bug.cgi?id=117152

Reviewed by Gustavo Noronha Silva.

* UIProcess/API/gtk/WebKitWebResource.cpp:
(resourceDataCallback):
(webkit_web_resource_get_data):
(webkit_web_resource_get_data_finish):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@151826 268f45cc-cd09-0410-ab3c-d52691b4dbfc