add default impl for context methods on shader

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 4af8f78bfbaa9ca8523b3262870081fbd10ace65..296a55b1f80c0d9052e4186f6ef1a8dae10f9bf5 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -207,16 +207,20 @@ public:
      *  Create the actual object that does the shading.
      *  Returns NULL if validContext() returns false.
      *  Size of storage must be >= contextSize.
+     *
+     *  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.
+     *
+     *  Base class implementation returns 0.

[scroggo, 2014/04/30 18:52:02]

Due to a requirement in SkSmallAllocator (which could be changed), returning 0
will fire an assert. If a class deliberately returns NULL for createContext,
they should return sizeof(Context) here.

      */
-    virtual size_t contextSize() const = 0;
+    virtual size_t contextSize() const;

[scroggo, 2014/04/30 18:52:02]

I like pure virtual function, because if someone writes a shader and overrides
createContext but forgets to override contextSize, they will fail to compile,
rather than get mysterious errors later.

 
     /**
      *  Helper to check the flags to know if it is legal to call shadeSpan16()