Should never be reached failure in WebCore::backgroundRectForBox

Should never be reached failure in WebCore::backgroundRectForBox
https://bugs.webkit.org/show_bug.cgi?id=150232

Reviewed by Simon Fraser.

We should never end up with simple container for composited layer when background-clip: text is present.
(not even when the box has no decoration to paint)

Source/WebCore:

Test: fast/backgrounds/background-clip-text-with-simple-container.html

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::paintsBoxDecorations):
(WebCore::RenderLayerBacking::isSimpleContainerCompositingLayer):
(WebCore::backgroundRectForBox):

LayoutTests:

* fast/backgrounds/background-clip-text-with-simple-container-expected.txt: Added.
* fast/backgrounds/background-clip-text-with-simple-container.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191680 268f45cc-cd09-0410-ab3c-d52691b4dbfc