Only store previous clip rects for PaintLayers that support subsequences.

third_party/WebKit/Source/core/paint/PaintLayer.h

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index c5ef7b90586085ef7cd3f9fad0d78cc727581450..9f7b688f966e28a0cf01ed73a286676a46edb5b4 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -584,6 +584,8 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
 
   bool PaintsWithTransform(GlobalPaintFlags) const;
 
+  bool SupportsSubsequenceCaching() const;
+
   // Returns true if background phase is painted opaque in the given rect.
   // The query rect is given in local coordinates.
   bool BackgroundIsKnownToBeOpaqueInRect(const LayoutRect&) const;
@@ -796,6 +798,10 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
     DCHECK(!needs_descendant_dependent_flags_update_);
     return has_descendant_with_clip_path_;
   }
+  bool HasDescendantThatSupportsSubsequenceCaching() const {
+    DCHECK(!needs_descendant_dependent_flags_update_);
+    return has_descendant_that_supports_subsequence_caching_;
+  }
 
   // Returns true if there is a descendant with blend-mode that is
   // not contained within another enclosing stacking context other
@@ -928,6 +934,9 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
   void SetPreviousPaintingClipRects(ClipRects& clip_rects) {
     previous_painting_clip_rects_ = &clip_rects;
   }
+  void ClearPreviousPaintingClipRects() {
+    previous_painting_clip_rects_.Clear();
+  }
 
   LayoutRect PreviousPaintDirtyRect() const {
     return previous_paint_dirty_rect_;
@@ -1221,6 +1230,7 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
   unsigned has_descendant_with_clip_path_ : 1;
   unsigned has_non_isolated_descendant_with_blend_mode_ : 1;
   unsigned has_ancestor_with_clip_path_ : 1;
+  unsigned has_descendant_that_supports_subsequence_caching_ : 1;
 
   unsigned self_painting_status_changed_ : 1;