Fix paint invalidation rect tracking within composited scrolling layers.

Source/core/layout/LayoutObject.h

Index: Source/core/layout/LayoutObject.h
diff --git a/Source/core/layout/LayoutObject.h b/Source/core/layout/LayoutObject.h
index 65b9b5e91d2fb9e1590eb0f5d74bda1739b2791f..c793b568376b7246b4adf71569920259a3a29944 100644
--- a/Source/core/layout/LayoutObject.h
+++ b/Source/core/layout/LayoutObject.h
@@ -1190,6 +1190,11 @@ protected:
     void setIsSlowRepaintObject(bool);
 
 private:
+    // Adjusts a paint invalidation rect in the space of |m_previousPaintInvalidationRect| and |m_previousPositionFromPaintInvalidationBacking|
+    // to be in the space of the |paintInvalidationContainer|,
+    // if needed. They can be different only if |paintInvalidationContainer| is a composited scroller.
+    void adjustInvalidationRectForCompositedScrolling(LayoutRect&, const LayoutBoxModelObject& paintInvalidationContainer);
+
     void clearLayoutRootIfNeeded() const;
 
     bool isInert() const;
@@ -1426,11 +1431,13 @@ private:
     // Store state between styleWillChange and styleDidChange
     static bool s_affectsParentBlock;
 
-    // This stores the paint invalidation rect from the previous frame.
+    // This stores the paint invalidation rect from the previous frame. This rect does *not* account for composited scrolling. See

[skobes, 2015/08/13 21:26:10]

It's a little unclear what it means to "account for composited scrolling". 
Could we just say that "the rect is in the coordinate space of the paint
invalidation container's scrolling contents layer"?

+    // adjustInvalidationRectForCompositedScrolling().
     LayoutRect m_previousPaintInvalidationRect;
 
     // This stores the position in the paint invalidation backing's coordinate.
     // It is used to detect layoutObject shifts that forces a full invalidation.
+    // This point does *not* account for composited scrolling. See adjustInvalidationRectForCompositedScrolling().
     LayoutPoint m_previousPositionFromPaintInvalidationBacking;
 };