Store previous painting clip rects on FragmentData.

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 7d808bdbffc4a6b75bc6e552cd3ad36ed4eb27a6..3b8eb8b6f8a6b02cf39ce47a1937f25b2ebf1514 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.h
@@ -1802,10 +1802,12 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
 
    protected:
     friend class PaintPropertyTreeBuilder;
+    friend class PrePaintTreeWalk;
     FRIEND_TEST_ALL_PREFIXES(AnimationCompositorAnimationsTest,
                              canStartAnimationOnCompositorTransformSPv2);
     FRIEND_TEST_ALL_PREFIXES(AnimationCompositorAnimationsTest,
                              canStartAnimationOnCompositorEffectSPv2);
+    FRIEND_TEST_ALL_PREFIXES(PrePaintTreeWalkTest, ClipRects);
 
     // The following non-const functions for ObjectPaintProperties should only
     // be called from PaintPropertyTreeBuilder.
@@ -1825,6 +1827,11 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
           fragment->ClearPaintProperties();
       }
     }
+    // Each LayoutObject has one or more painting fragments (exactly one

[wkorman, 2017/05/01 21:58:09]

Not for this change, but to consider -- fragment data is currently stored in
RareData but it sounds like it's not actually very rare any more.

Perhaps we should move it to a straight member data.

[chrishtr, 2017/05/01 22:40:30]

FYI it's only for PaintLayers that it is set, so it is somewhat rare.

+    // in the absence of multicol/pagination).
+    // See ../paint/README.md for more on fragments.
+    FragmentData* FirstFragment();
+    FragmentData& EnsureFirstFragment();
 
     // The following non-const functions for local border box properties should
     // only be called from PaintPropertyTreeBuilder.