Allow clients of the WKRenderLayerRef C SPI to connect layer trees across frame boundaries
https://bugs.webkit.org/show_bug.cgi?id=139021

Reviewed by Dan Bernstein.

Add WKRenderLayerGetFrameContentsLayer which returns a WKRenderLayerRef for
the content document of a RenderWidget, if its has one (e.g. frames, iframes
and <object> with document contents).

* Shared/API/c/WKRenderLayer.cpp:
(WKRenderLayerGetFrameContentsLayer):
* Shared/API/c/WKRenderLayer.h:
* Shared/UserMessageCoders.h:
(WebKit::UserMessageEncoder::baseEncode):
(WebKit::UserMessageDecoder::baseDecode):
* Shared/WebRenderLayer.cpp:
(WebKit::WebRenderLayer::create):
(WebKit::WebRenderLayer::WebRenderLayer):
* Shared/WebRenderLayer.h:
(WebKit::WebRenderLayer::frameContentsLayer):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176568 268f45cc-cd09-0410-ab3c-d52691b4dbfc