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/SkImageRef.cpp b/src/images/SkImageRef.cpp
index cdd80e7..1d6b270 100644
--- a/src/images/SkImageRef.cpp
+++ b/src/images/SkImageRef.cpp
@@ -189,7 +189,7 @@
     fFactory = NULL;
 }
 
-void SkImageRef::flatten(SkFlattenableWriteBuffer& buffer) {
+void SkImageRef::flatten(SkFlattenableWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
 
     buffer.write8(fConfig);