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
54924243c1