Refactoring RenderLayerCompositor to avoid redundant checks for Compositing reasons

Source/core/rendering/RenderLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1556 matching lines @@
 }
 
 CompositingReasons RenderLayerCompositor::directReasonsForCompositing(const RenderLayer* layer) const
 {
     RenderObject* renderer = layer->renderer();
     CompositingReasons directReasons = CompositingReasonNone;
 
     if (requiresCompositingForTransform(renderer))
         directReasons |= CompositingReason3DTransform;
 
+    // Only zero or one of the following conditions will be true for a given RenderLayer.
     if (requiresCompositingForVideo(renderer))
         directReasons |= CompositingReasonVideo;
-
-    if (requiresCompositingForCanvas(renderer))
+    else if (requiresCompositingForCanvas(renderer))
         directReasons |= CompositingReasonCanvas;
-
-    if (requiresCompositingForPlugin(renderer))
+    else if (requiresCompositingForPlugin(renderer))
         directReasons |= CompositingReasonPlugin;
-
-    if (requiresCompositingForFrame(renderer))
+    else if (requiresCompositingForFrame(renderer))
         directReasons |= CompositingReasonIFrame;
     
     if (requiresCompositingForBackfaceVisibilityHidden(renderer))
         directReasons |= CompositingReasonBackfaceVisibilityHidden;
 
     if (requiresCompositingForAnimation(renderer))
         directReasons |= CompositingReasonAnimation;
 
     if (requiresCompositingForTransition(renderer))
         directReasons |= CompositingReasonAnimation;
@@ skipping 1155 matching lines @@
     info.addMember(m_layerForScrollCorner, "layerForScrollCorner");
 #if ENABLE(RUBBER_BANDING)
     info.addMember(m_layerForOverhangAreas, "layerForOverhangAreas");
     info.addMember(m_contentShadowLayer, "contentShadowLayer");
     info.addMember(m_layerForTopOverhangArea, "layerForTopOverhangArea");
     info.addMember(m_layerForBottomOverhangArea, "layerForBottomOverhangArea");
 #endif
 }
 
 } // namespace WebCore