[Qt][DRT] Add support for overriding the "WebKitDisplayImagesKey" preference.
https://bugs.webkit.org/show_bug.cgi?id=98200

Patch by Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com> on 2012-10-03
Reviewed by Csaba Osztrogonác.

Tools:

* DumpRenderTree/qt/TestRunnerQt.cpp:
(TestRunner::overridePreference):

LayoutTests:

* platform/qt/TestExpectations: Unskip a few tests that depended
on this feature.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@130271 268f45cc-cd09-0410-ab3c-d52691b4dbfc