https://bugs.webkit.org/show_bug.cgi?id=60421

Custom scrollbars nested inside an overlay scrollbar overflow region end up 
painting twice
-and corresponding-
<rdar://problem/9389072>

Reviewed by Darin Adler.

Source/WebCore: 

Be sure not to paint custom css scrollbars during the special overlay-scrollbar 
painting pass.
* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::paintOverflowControls):

LayoutTests: 

* platform/mac/scrollbars/scrollbars-on-positioned-content-expected.png: Added.
* platform/mac/scrollbars/scrollbars-on-positioned-content-expected.txt: Added.
* scrollbars/scrollbars-on-positioned-content.html: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@85990 268f45cc-cd09-0410-ab3c-d52691b4dbfc