Hoist compositorDrivenAcceleratedScrollingEnabled check out of scrollParent

Source/core/rendering/RenderLayer.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 1706 matching lines @@
 
 void RenderLayer::convertToLayerCoords(const RenderLayer* ancestorLayer, LayoutRect& rect) const
 {
     LayoutPoint delta;
     convertToLayerCoords(ancestorLayer, delta);
     rect.move(-delta.x(), -delta.y());
 }
 
 RenderLayer* RenderLayer::scrollParent() const
 {
-    if (!renderer()->compositorDrivenAcceleratedScrollingEnabled())
-        return 0;
+    ASSERT(renderer()->compositorDrivenAcceleratedScrollingEnabled());
 
     // Normal flow elements will be parented under the main scrolling layer, so
     // we don't need a scroll parent/child relationship to get them to scroll.
     if (stackingNode()->isNormalFlowOnly())
         return 0;
 
     // We should never have an ancestor dependent property cache outside of the
     // compositing update phase.
     ASSERT(isInCompositingUpdate() || !m_ancestorDependentPropertyCache);
 
@@ skipping 2441 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif