Reviewed by Eric Seidel and Xan Lopez.

        [Gtk] Enable http/tests/appcache tests
        https://bugs.webkit.org/show_bug.cgi?id=27674

        * GNUmakefile.am: Add webkit/webkitapplicationcache.cpp

        Enable passing http/tests/appcache tests.

        * platform/gtk/Skipped:

        Add 'enable-offline-web-application-cache' WebSetting to enable
        or disable the appcache. Also, Add appcache SPI for DRT to use.

        * webkit/webkitapplicationcache.cpp: Added.
        (webkit_application_cache_set_maximum_size):
        * webkit/webkitprivate.cpp:
        (webkit_init):
        * webkit/webkitprivate.h:
        * webkit/webkitwebsettings.cpp:
        (webkit_web_settings_class_init):
        (webkit_web_settings_set_property):
        (webkit_web_settings_get_property):
        (webkit_web_settings_copy):
        * webkit/webkitwebview.cpp:
        (webkit_web_view_update_settings):
        (webkit_web_view_settings_notif...