Implement -webkit-box-reflect as a filter.

third_party/WebKit/Source/core/layout/LayoutObject.h

Index: third_party/WebKit/Source/core/layout/LayoutObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.h b/third_party/WebKit/Source/core/layout/LayoutObject.h
index 92f100e0d222578fdcf01c60a1daf75fd5502ee1..47b2e1c1bf113e8f4ed0f39c9bf7bfde5ab0ab68 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.h
@@ -720,6 +720,10 @@ public:
 
     bool hasFilter() const { return style() && style()->hasFilter(); }

[pdr., 2016/03/29 18:36:55]

Would it be cleaner to update paintInvalidationState::supportsCachedOffsets to
call "hasFilterInducingProperty" too?

With that change, I think we can remove hasFilter entirely. I think this will
keep us from making mistakes in the future.

     bool hasBackdropFilter() const { return style() && style()->hasBackdropFilter(); }
+    bool hasFilterInducingProperty() const

[Stephen White, 2016/03/30 14:41:35]

I find this name unintuitive. Could we be more explicit, e.g.,
hasFilterOrReflection()?

Think about it from the point-of-view of someone new to this code: what is a
filter-inducing property? Is this a fundamental property or an implementation
detail? You could explain it with a comment, but Blink style is to put it in the
name.

+    {
+        return (style() && style()->hasFilter()) || (RuntimeEnabledFeatures::cssBoxReflectFilterEnabled() && hasReflection());
+    }
 
     bool hasShapeOutside() const { return style() && style()->shapeOutside(); }
 
@@ -1264,7 +1268,7 @@ public:
     bool shouldUseTransformFromContainer(const LayoutObject* container) const;
     void getTransformFromContainer(const LayoutObject* container, const LayoutSize& offsetInContainer, TransformationMatrix&) const;
 
-    bool createsGroup() const { return isTransparent() || hasMask() || hasFilter() || style()->hasBlendMode(); }
+    bool createsGroup() const { return isTransparent() || hasMask() || hasFilterInducingProperty() || style()->hasBlendMode(); }
 
     // Collects rectangles that the outline of this object would be drawing along the outside of,
     // even if the object isn't styled with a outline for now. The rects also cover continuations.