Reviewed by Sam.

    Reviewed by Sam.

        - <rdar://4516170> Back/Forward Cache on Windows

        * WebFrame.cpp:
        (WebFrame::forceLayout): Implemented this. Just calling through to
        Frame::forceLayout matches the Mac pretty well and should be enough
        for now. Without this we won't repaint the new page when we go back
        and forward.
        (WebFrame::dispatchWillSubmitForm): Use COMPtr's AdoptCOM constructor
        instead of the adoptRef function.
        (WebFrame::restoreViewState): Removed unneeded notImplemented() call.
        This is implemented as much as it needs to be.
        (WebFrame::shouldGoToHistoryItem): Ditto.
        (WebFrame::saveViewStateToItem): Ditto.
        (WebFrame::canCachePage): Ditto, also changed it to return true. This
        is what was preventing the back/forward cache from running, and there's
        no need for any additional checks in the Windows version.

        * WebView.cpp:
        (PreferencesChangedOrRemovedObserver::onNotify): Fix backwards null check
        that was preventing this notification from ever being sent, which in turn
        meant that page cache had a capacity of 0.
        (WebView::initWithFrame): Removed unneeded setUsesPageCache(false) code.
        This actually was ineffective and ended up doing no harm, but we don't
        need it.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@28822 268f45cc-cd09-0410-ab3c-d52691b4dbfc