Merge 141160

Merge 141160
> REGRESSION (r135628-135632): Double box shadow failure to render
> https://bugs.webkit.org/show_bug.cgi?id=107833
> 
> Reviewed by Simon Fraser.
> 
> Source/WebCore:
> 
> Regression caused by http://trac.webkit.org/changeset/135629
> The regression was due to faulty occlusion logic that was assuming
> that drawing the background color of a render box background layer
> could be skipped when the same layer also has an opaque image attached.
> In the case where the background color is drawn for the purpose of
> rendering a box shadow, the shadow is typically not
> completely occluded by the background image because of the shadow
> blur and/or offset.  This patch fixes the problem by not culling a
> background draw if it is used to draw a box shadow.
> 
> Test: fast/backgrounds/gradient-background-shadow.html
> 
> * rendering/RenderBoxModelObject.cpp:
> (WebCore::RenderBoxModelObject::paintFillLayerExtended):
> Changing occlusion culling test to never cull background color
> draw if it is used to draw a box shadow. This is because box shadows
> can draw outside the border fill region.
> 
> LayoutTests:
> 
> New ref test verifies that box shadow is drawn when
> background is an opaque image. Test uses an blue gradient
> as background image. Reference uses blue background color.
> 
> * fast/backgrounds/gradient-background-shadow-expected.html: Added.
> * fast/backgrounds/gradient-background-shadow.html: Added.
> 

TBR=junov@google.com
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=141267