add default impl for context methods on shader

include/core/SkShader.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkShader_DEFINED
 #define SkShader_DEFINED
@@ skipping 189 matching lines @@
      *  Subclasses should be sure to call their INHERITED::validContext() if
      *  they override this method.
      */
     virtual bool validContext(const SkBitmap& device, const SkPaint& paint,
                               const SkMatrix& matrix, SkMatrix* totalInverse = NULL) const;
 
     /**
      *  Create the actual object that does the shading.
      *  Returns NULL if validContext() returns false.
      *  Size of storage must be >= contextSize.
+     *  Your subclass must also override contextSize() if it overrides createContext().
+     *
+     *  Base class implementation returns NULL.
      */
     virtual Context* createContext(const SkBitmap& device,
                                    const SkPaint& paint,
                                    const SkMatrix& matrix,
-                                   void* storage) const = 0;
+                                   void* storage) const;
 
     /**
      *  Return the size of a Context returned by createContext.
+     *
+     *  Override this if your subclass overrides createContext, to return the correct size of
+     *  your subclass' context.
      */
-    virtual size_t contextSize() const = 0;
+    virtual size_t contextSize() const;
 
     /**
      *  Helper to check the flags to know if it is legal to call shadeSpan16()
      */
     static bool CanCallShadeSpan16(uint32_t flags) {
         return (flags & kHasSpan16_Flag) != 0;
     }
 
     /**
      Gives method bitmap should be read to implement a shader.
@@ skipping 166 matching lines @@
 
 private:
     SkMatrix            fLocalMatrix;
 
     bool computeTotalInverse(const SkMatrix& matrix, SkMatrix* totalInverse) const;
 
     typedef SkFlattenable INHERITED;
 };
 
 #endif