[GTK] Allow sharing the WebCore include list with the Chromium build
https://bugs.webkit.org/show_bug.cgi?id=110241

Patch by Martin Robinson <mrobinson@igalia.com> on 2013-02-23
Reviewed by Dirk Pranke.

* WebCore.gyp/WebCore.gyp: Moved shared include directories to
WebCore.gypi.
* WebCore.gyp/WebCoreGTK.gyp: Added. A skeleton gyp file for WebCoreGTK+.
* WebCore.gypi: Added shared include directories.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143844 268f45cc-cd09-0410-ab3c-d52691b4dbfc