Cleanup code related to SkFlattenable::flatten()

The following changes were made by this CL:

1. Make flatten() a protected method as callers should use
   SkFlattenableWriteBuffer to flatten an object
2. Make flatten a const method (including subclasses)
3. Mark subclass implementation of flatten with SK_OVERRIDE
4. Ensure overridden flatten impls call their parent
5. Remove no-op implementations of flatten from subclasses

Additionally, if necessary the unflattening constructor was
also moved to the protected section of the subclasses
header if it was not already there.

git-svn-id: http://skia.googlecode.com/svn/trunk@3540 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/effects/SkLayerRasterizer.h b/include/effects/SkLayerRasterizer.h
index e31ed33..c51b174 100644
--- a/include/effects/SkLayerRasterizer.h
+++ b/include/effects/SkLayerRasterizer.h
@@ -32,13 +32,11 @@
 	*/
     void addLayer(const SkPaint& paint, SkScalar dx, SkScalar dy);
 
-    // overrides from SkFlattenable
-    virtual void    flatten(SkFlattenableWriteBuffer&);
-
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkLayerRasterizer)
 
 protected:
     SkLayerRasterizer(SkFlattenableReadBuffer&);
+    virtual void flatten(SkFlattenableWriteBuffer&) const SK_OVERRIDE;
 
     // override from SkRasterizer
     virtual bool onRasterize(const SkPath& path, const SkMatrix& matrix,