Partly revert of Conditionally create PaintLayer's scrollable area object

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1451 matching lines @@
 {
     ASSERT(!m_stackingNode);
     if (requiresStackingNode())
         m_stackingNode = adoptPtr(new PaintLayerStackingNode(this));
     else
         m_stackingNode = nullptr;
 }
 
 void PaintLayer::updateScrollableArea()
 {
-    if (requiresScrollableArea()) {
-        if (!m_scrollableArea)
-            m_scrollableArea = PaintLayerScrollableArea::create(*this);
-    } else if (m_scrollableArea) {
-        if (m_scrollableArea)
-            m_scrollableArea->dispose();
-        m_scrollableArea.clear();
-    }
+    ASSERT(!m_scrollableArea);
+    if (requiresScrollableArea())
+        m_scrollableArea = PaintLayerScrollableArea::create(*this);
 }
 
 bool PaintLayer::hasOverflowControls() const
 {
     return m_scrollableArea && (m_scrollableArea->hasScrollbar() || m_scrollableArea->scrollCorner() || layoutObject()->style()->resize() != RESIZE_NONE);
 }
 
 void PaintLayer::appendSingleFragmentIgnoringPagination(PaintLayerFragments& fragments, const PaintLayer* rootLayer, const LayoutRect& dirtyRect, ClipRectsCacheSlot clipRectsCacheSlot, OverlayScrollbarSizeRelevancy inOverlayScrollbarSizeRelevancy, ShouldRespectOverflowClip respectOverflowClip, const LayoutPoint* offsetFromRoot, const LayoutSize& subPixelAccumulation)
 {
     PaintLayerFragment fragment;
@@ skipping 1085 matching lines @@
 }
 
 void PaintLayer::styleChanged(StyleDifference diff, const ComputedStyle* oldStyle)
 {
     if (attemptDirectCompositingUpdate(diff, oldStyle))
         return;
 
     m_stackingNode->updateIsTreatedAsStackingContext();
     m_stackingNode->updateStackingNodesAfterStyleChange(oldStyle);
 
-    updateScrollableArea();
     if (m_scrollableArea)
         m_scrollableArea->updateAfterStyleChange(oldStyle);
 
     // Overlay scrollbars can make this layer self-painting so we need
     // to recompute the bit once scrollbars have been updated.
     updateSelfPaintingLayer();
 
     if (!oldStyle || !layoutObject()->style()->reflectionDataEquivalent(oldStyle)) {
         ASSERT(!oldStyle || diff.needsFullLayout());
         updateReflectionInfo(oldStyle);
@@ skipping 246 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif