Reviewed by Darin Fisher.

        Provide an API for querying whether a WebWidget is using gpu accelerated compositing.
        Also changed: isAcceleratedCompositing to: isAcceleratedCompositingActive.
        https://bugs.webkit.org/show_bug.cgi?id=38220

        * public/WebWidget.h:
        * src/WebPopupMenuImpl.h:
        (WebKit::WebPopupMenuImpl::isAcceleratedCompositingActive):
        * src/WebViewImpl.cpp:
        (WebKit::WebViewImpl::WebViewImpl):
        (WebKit::WebViewImpl::paint):
        (WebKit::WebViewImpl::isAcceleratedCompositingActive):
        (WebKit::WebViewImpl::setRootGraphicsLayer):
        (WebKit::WebViewImpl::setIsAcceleratedCompositingActive):
        (WebKit::WebViewImpl::updateRootLayerContents):
        * src/WebViewImpl.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58793 268f45cc-cd09-0410-ab3c-d52691b4dbfc