[WK2] Use is<>() / downcast<>() for DrawingArea subclasses
https://bugs.webkit.org/show_bug.cgi?id=137497

Reviewed by Benjamin Poulain.

Use is<>() / downcast<>() for DrawingArea subclasses.

* WebProcess/WebPage/DrawingArea.h:
* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::didCommitLoad):
* WebProcess/WebPage/ios/WebPageIOS.mm:
(WebKit::WebPage::synchronizeDynamicViewportUpdate):
* WebProcess/WebPage/mac/RemoteLayerTreeDrawingArea.h:
* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174413 268f45cc-cd09-0410-ab3c-d52691b4dbfc