[EFL][CoordinatedGraphics] All EFL layout tests are broken since r174231
https://bugs.webkit.org/show_bug.cgi?id=137443

Reviewed by Zoltan Herczeg.

EFL layout test and MiniBrowser have broken since r174231. r174231 was to move PageOverlay implementation
from WebKit2 to WebCore with lazy initializing of PageOverlayController. It caused all EFL layout tests
crash and build break. To restore EFL layout test, this patch sets viewOverlayRootLayer through
attachViewOverlayGraphicsLayer() as mac port's one in r174231.

* WebProcess/WebPage/CoordinatedGraphics/CoordinatedDrawingArea.cpp:
(WebKit::CoordinatedDrawingArea::attachViewOverlayGraphicsLayer):
* WebProcess/WebPage/CoordinatedGraphics/CoordinatedDrawingArea.h:
* WebProcess/WebPage/CoordinatedGraphics/CoordinatedLayerTreeHost.cpp:
(WebKit::CoordinatedLayerTreeHost::CoordinatedLayerTreeHost):
(WebKit::CoordinatedLayerTreeHost::updateRootLayers):
(WebKit::CoordinatedLayerTreeHost::setViewOverlayRootLayer):
(WebKit::CoordinatedLayerTreeHost::setRootCompositingLayer):
(WebKit::CoordinatedLayerTreeHost::didFlushRootLayer):
* WebProcess/WebPage/CoordinatedGraphics/CoordinatedLayerTreeHost.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174435 268f45cc-cd09-0410-ab3c-d52691b4dbfc