Fix leak in WeakPtrTest.MoveOwnershipAfterInvalidate

Needs a virtual destructor to clean up correctly when passing
a derived TargetWithFactory instance to the background thread for
deletion (BackgroundThread::DeleteTarget gets a base class pointer).

BUG=268267

Review URL: https://codereview.chromium.org/100743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250653 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250653