[GTK] Fix build when DRAG_SUPPORT is disabled
https://bugs.webkit.org/show_bug.cgi?id=137361

Patch by Lorenzo Tilve <ltilve@igalia.com> on 2014-10-05
Reviewed by Carlos Garcia Campos.

Flag out unbuildable code without DRAG_SUPPORT.

Source/WebCore:

* platform/gtk/GtkDragAndDropHelper.cpp:
* platform/gtk/GtkDragAndDropHelper.h:

Source/WebKit2:

* Shared/gtk/ArgumentCodersGtk.cpp:
* Shared/gtk/ArgumentCodersGtk.h:
* UIProcess/API/gtk/PageClientImpl.cpp:
* UIProcess/API/gtk/PageClientImpl.h:
* UIProcess/API/gtk/WebKitWebViewBase.cpp:
(webkitWebViewBaseConstructed):
* UIProcess/PageClient.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174328 268f45cc-cd09-0410-ab3c-d52691b4dbfc