[Qt][WTR] Fix the build on Mac after r149692

Unreviewed build fix.

* WebKitTestRunner/InjectedBundle/InjectedBundlePage.cpp:
(WTR::InjectedBundlePage::dumpDOMAsWebArchive):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149731 268f45cc-cd09-0410-ab3c-d52691b4dbfc