Never remove root compositing layers in the web process

Never remove root compositing layers in the web process
https://bugs.webkit.org/show_bug.cgi?id=82255
<rdar://problem/11058521>

Reviewed by Sam Weinig.

Since we never leave accelerated compositing mode when using tiled drawing, we should
never remove root compositing layers in the web process.

* WebProcess/WebPage/mac/TiledCoreAnimationDrawingArea.mm:
(WebKit::TiledCoreAnimationDrawingArea::setRootCompositingLayer):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@112175 268f45cc-cd09-0410-ab3c-d52691b4dbfc