Simplify computation of the invalidation rect for a frame.

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

Index: third_party/WebKit/Source/core/paint/PaintLayerClipper.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerClipper.h b/third_party/WebKit/Source/core/paint/PaintLayerClipper.h
index e8339b2eb3eaaee1d65b91d1e33359d2a29ef21a..468ef61a0ca82393180b5f86e8edf8faef22ff0e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerClipper.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayerClipper.h
@@ -147,7 +147,7 @@ private:
 // #container and #fixed are siblings in the paint tree but #container does
 // clip #fixed. This is the reason why we compute the painting clip rects during
 // a layout tree walk and cache them for painting.
-class PaintLayerClipper {
+class CORE_EXPORT PaintLayerClipper {
     DISALLOW_NEW();
     WTF_MAKE_NONCOPYABLE(PaintLayerClipper);
 public:
@@ -156,7 +156,7 @@ public:
     void clearClipRectsIncludingDescendants();
     void clearClipRectsIncludingDescendants(ClipRectsCacheSlot);
 
-    LayoutRect childrenClipRect() const; // Returns the foreground clip rect of the layer in the document's coordinate space.
+    LayoutRect childrenClipRect() const; // Returns the foreground clip rect of the layer in the absolute coordinate space.
     LayoutRect localClipRect() const; // Returns the background clip rect of the layer in the local coordinate space.
 
     ClipRect backgroundClipRect(const ClipRectsContext&) const;