:before content incorrectly placed in continuation

:before content incorrectly placed in continuation
when we don't have a first child.
https://bugs.webkit.org/show_bug.cgi?id=78380

Reviewed by David Hyatt.

Source/WebCore:

Test: fast/css-generated-content/before-content-continuation-chain.html

* rendering/RenderObjectChildList.cpp:
(WebCore::RenderObjectChildList::updateBeforeAfterContent):

LayoutTests:

* fast/css-generated-content/before-content-continuation-chain-expected.png: Added.
* fast/css-generated-content/before-content-continuation-chain-expected.txt: Added.
* fast/css-generated-content/before-content-continuation-chain.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108109 268f45cc-cd09-0410-ab3c-d52691b4dbfc