Reviewed by Dan Bernstein.

        Get rid of PageClient::isPageVisible and pass visibility directly in setPageIsVisible
        https://bugs.webkit.org/show_bug.cgi?id=38493

        * UIProcess/API/mac/PageClientImpl.h:
        * UIProcess/API/mac/PageClientImpl.mm:
        Remove isPageVisible.
        
        * UIProcess/API/mac/WKView.mm:
        (isViewVisible):
        New function (moved here from PageClientImpl).
        
        (-[WKView _updateVisibility]):
        Call didChangeVisibility.
        
        (-[WKView viewDidMoveToWindow]):
        (-[WKView viewDidHide]):
        (-[WKView viewDidUnhide]):
        Call _updateVisibility.
        
        * UIProcess/DrawingAreaProxy.h:
        Rename didChangeVisibility to setPageIsVisible and add an isVisible parameter.
        
        * UIProcess/DrawingAreaProxyUpdateChunk.cpp:
        (WebKit::DrawingAreaProxyUpdateChunk::setPageIsVisible):
        Don't call WebPageProxy::isVisible.

        * UIProcess/DrawingAreaProxyUpdateChunk.h:
        
        * UIProcess/PageClient.h:
        Remove isPageVisible.

        * UIProcess/WebPageProxy.cpp:
        * UIProcess/WebPageProxy.h:
        Remove isVisible.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58714 268f45cc-cd09-0410-ab3c-d52691b4dbfc