[chromium] Move chromium compositor implementation files into separate section in...

[chromium] Move chromium compositor implementation files into separate section in WebCore.gypi
https://bugs.webkit.org/show_bug.cgi?id=90201

Patch by James Robinson <jamesr@chromium.org> on 2012-06-28
Reviewed by Adam Barth.

This moves the chromium compositor implementation files to a separate gyp variable to make future changes
easier. The files still all link into webcore_platform.lib, as before.

* WebCore.gyp/WebCore.gyp:
* WebCore.gypi:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121473 268f45cc-cd09-0410-ab3c-d52691b4dbfc