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/samplecode/SampleText.cpp b/samplecode/SampleText.cpp
index 7ce1abe..7ed16dc 100644
--- a/samplecode/SampleText.cpp
+++ b/samplecode/SampleText.cpp
@@ -131,11 +131,6 @@
 
     typedef SkFlattenable* (*Factory)(SkFlattenableReadBuffer&);
 
-    // overrides for SkFlattenable
-    virtual void flatten(SkFlattenableWriteBuffer& b) {
-    //    this->INHERITED::flatten(b);  How can we know if this is legal????
-        b.write32(SkScalarToFixed(fExp));
-    }
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkPowerMode)
 
 private:
@@ -143,10 +138,14 @@
     uint8_t fTable[256];    // cache
 
     void init(SkScalar exponent);
-    SkPowerMode(SkFlattenableReadBuffer& b) : SkXfermode(b) {
+    SkPowerMode(SkFlattenableReadBuffer& b) : INHERITED(b) {
         // read the exponent
         this->init(SkFixedToScalar(b.readS32()));
     }
+    virtual void flatten(SkFlattenableWriteBuffer& b) const SK_OVERRIDE {
+        this->INHERITED::flatten(b);
+        b.write32(SkScalarToFixed(fExp));
+    }
 
     typedef SkXfermode INHERITED;
 };