Fix a memory leak introduced in r149692
https://bugs.webkit.org/show_bug.cgi?id=115766

Patch by Alex Christensen <achristensen@apple.com> on 2013-05-07
Reviewed by Mark Rowe.

In r149692, the fix for <http://webkit.org/b/42324>, a call to WKBundleFrameCopyWebArchive was added without any
matching call to WKRelease. An earlier attempted fix in r149697 introduced a RetainPtr but failed to adopt the object.

* WebKitTestRunner/InjectedBundle/InjectedBundlePage.cpp:
(WTR::InjectedBundlePage::dumpDOMAsWebArchive):
Fix the memory leak by switching to WKRetainPtr and adopting the returned object.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149704 268f45cc-cd09-0410-ab3c-d52691b4dbfc