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/src/images/SkFlipPixelRef.cpp b/src/images/SkFlipPixelRef.cpp
index 768a67d..6ab5bbc 100644
--- a/src/images/SkFlipPixelRef.cpp
+++ b/src/images/SkFlipPixelRef.cpp
@@ -60,7 +60,7 @@
     fMutex.release();
 }
 
-void SkFlipPixelRef::flatten(SkFlattenableWriteBuffer& buffer) {
+void SkFlipPixelRef::flatten(SkFlattenableWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     
     buffer.write32(fSize);