Reviewed by Sam Weinig.

        https://bugs.webkit.org/show_bug.cgi?id=38415
        Have the WKView notify the DrawingAreaProxy when its visibility changes.

        * UIProcess/API/mac/PageClientImpl.h:
        * UIProcess/API/mac/PageClientImpl.mm:
        (WebKit::PageClientImpl::isPageVisible):
        * UIProcess/API/mac/WKView.mm:
        (-[WKView viewDidMoveToWindow]):
        (-[WKView viewDidHide]):
        (-[WKView viewDidUnhide]):
        * UIProcess/DrawingAreaProxy.h:
        * UIProcess/DrawingAreaProxyUpdateChunk.cpp:
        (WebKit::DrawingAreaProxyUpdateChunk::DrawingAreaProxyUpdateChunk):
        (WebKit::DrawingAreaProxyUpdateChunk::didChangeVisibility):
        * UIProcess/DrawingAreaProxyUpdateChunk.h:
        * UIProcess/PageClient.h:
        * UIProcess/WebPageProxy.cpp:
        (WebKit::WebPageProxy::isVisible):
        * UIProcess/WebPageProxy.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58688 268f45cc-cd09-0410-ab3c-d52691b4dbfc