Simplify flattening to just write enough to call the factory

include/effects/SkComposeImageFilter.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkComposeImageFilter_DEFINED
 #define SkComposeImageFilter_DEFINED
 
 #include "SkImageFilter.h"
 
 class SK_API SkComposeImageFilter : public SkImageFilter {
 public:
     virtual ~SkComposeImageFilter();
 
-    static SkComposeImageFilter* Create(SkImageFilter* outer, SkImageFilter* inner) {
+    static SkImageFilter* Create(SkImageFilter* outer, SkImageFilter* inner) {
+        if (NULL == outer) {
+            return SkSafeRef(inner);
+        }
+        if (NULL == inner) {
+            return SkRef(outer);
+        }
         SkImageFilter* inputs[2] = { outer, inner };
         return SkNEW_ARGS(SkComposeImageFilter, (inputs));
     }
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkComposeImageFilter)
 
 protected:
-    explicit SkComposeImageFilter(SkImageFilter* inputs[2]) : INHERITED(2, inputs) {}
+    explicit SkComposeImageFilter(SkImageFilter* inputs[2]) : INHERITED(2, inputs) {
+        SkASSERT(inputs[0]);
+        SkASSERT(inputs[1]);
+    }
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
     explicit SkComposeImageFilter(SkReadBuffer& buffer);
+#endif
 
     virtual bool onFilterImage(Proxy*, const SkBitmap& src, const Context&,
                                SkBitmap* result, SkIPoint* loc) const SK_OVERRIDE;
     virtual bool onFilterBounds(const SkIRect&, const SkMatrix&, SkIRect*) const SK_OVERRIDE;
 
 private:
     typedef SkImageFilter INHERITED;
 };
 
 #endif