Unifies ScrollView and RenderLayer to use non-shifted [minPos, maxPos] scroll offset

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index cb4710c8df9f4617f79c239035241f159278ea7c..3a42657eadf36e750eefed35eeaed4e86518c6cf 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -2579,8 +2579,13 @@ IntPoint RenderLayer::minimumScrollPosition() const
 
 IntPoint RenderLayer::maximumScrollPosition() const
 {
-    // FIXME: m_scrollSize may not be up-to-date if m_scrollDimensionsDirty is true.
-    return -scrollOrigin() + roundedIntSize(m_scrollSize) - visibleContentRect(IncludeScrollbars).size();
+    RenderBox* box = renderBox();
+    if (!box)
+        return -scrollOrigin();
+
+    LayoutRect overflowRect(box->layoutOverflowRect());
+    box->flipForWritingMode(overflowRect);
+    return -scrollOrigin() + enclosingIntRect(overflowRect).size() - enclosingIntRect(box->clientBoxRect()).size();
 }
 
 IntRect RenderLayer::visibleContentRect(VisibleContentRectIncludesScrollbars scrollbarInclusion) const