Reviewed by Adam Barth.

        Add a new setting which gives the option of assigning every file:///
        to its own unique domain.

        https://bugs.webkit.org/show_bug.cgi?id=34778

        Test: Pending in forthcoming separate change due to non-trivial
        dependencies.

        * dom/Document.cpp:
        (WebCore::Document::initSecurityContext): Place file:/// URI documents
        into their own unique domains if the new setting requires it.
        * page/Settings.h:
        * page/Settings.cpp:
        (WebCore::Settings::setAllowFileAccessFromFileURLs): Simple setter.
        * page/SecurityOrigin.h:
        * page/SecurityOrigin.cpp:
        (WebCore::SecurityOrigin::makeUnique): Add simple ability to force an
        origin to be considered unique.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@54873 268f45cc-cd09-0410-ab3c-d52691b4dbfc