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/core/SkShader.cpp b/src/core/SkShader.cpp
index a3b3b6a..bfdebc1 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -41,7 +41,7 @@
     SkDEBUGCODE(fInSession = false;)
 }
 
-void SkShader::flatten(SkFlattenableWriteBuffer& buffer) {
+void SkShader::flatten(SkFlattenableWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     buffer.writeBool(fLocalMatrix != NULL);
     if (fLocalMatrix) {
@@ -238,7 +238,7 @@
     fColor = b.readU32();
 }
 
-void SkColorShader::flatten(SkFlattenableWriteBuffer& buffer) {
+void SkColorShader::flatten(SkFlattenableWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);
     buffer.write8(fInheritColor);
     if (fInheritColor) {
@@ -331,8 +331,6 @@
 
 #include "SkEmptyShader.h"
 
-SkEmptyShader::SkEmptyShader(SkFlattenableReadBuffer& b) : INHERITED(b) {}
-
 uint32_t SkEmptyShader::getFlags() { return 0; }
 uint8_t SkEmptyShader::getSpan16Alpha() const { return 0; }
 
@@ -351,8 +349,4 @@
     SkDEBUGFAIL("should never get called, since setContext() returned false");
 }
 
-void SkEmptyShader::flatten(SkFlattenableWriteBuffer& buffer) {
-    this->INHERITED::flatten(buffer);
-}
-
 SK_DEFINE_FLATTENABLE_REGISTRAR(SkEmptyShader)