Don't let -[CALayer renderInContext:] try to render WebView's root layer

Don't let -[CALayer renderInContext:] try to render WebView's root layer
https://bugs.webkit.org/show_bug.cgi?id=86773
<rdar://problem/10950075>

Reviewed by Dan Bernstein.

-[CALayer renderInContext:] doesn't correctly render our layer tree, so
don't let it try. Calling drawRect: with a bitmap context will already
do the right thing for WebViews.

* WebView/WebHTMLView.mm: Create a subclass of CALayer called WebLayerHostingLayer.
(-[WebLayerHostingLayer renderInContext:]): override renderInContext: to be a NOOP.
(-[WebHTMLView attachRootLayer:]): Create a WebLayerHostingLayer rather than a CALayer.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@117510 268f45cc-cd09-0410-ab3c-d52691b4dbfc