Revert of Revert of Extract most of the mutable state of SkShader into a separate Context object.

src/effects/gradients/SkRadialGradient.h

Index: src/effects/gradients/SkRadialGradient.h
diff --git a/src/effects/gradients/SkRadialGradient.h b/src/effects/gradients/SkRadialGradient.h
index 4a725145b030515756ee0b06453e1f22f750c92a..a3d04b1aaf0c4f0deaad00d8ad1e88292c068889 100644
--- a/src/effects/gradients/SkRadialGradient.h
+++ b/src/effects/gradients/SkRadialGradient.h
@@ -14,10 +14,24 @@
 class SkRadialGradient : public SkGradientShaderBase {
 public:
     SkRadialGradient(const SkPoint& center, SkScalar radius, const Descriptor&);
-    virtual void shadeSpan(int x, int y, SkPMColor* dstC, int count)
-        SK_OVERRIDE;
-    virtual void shadeSpan16(int x, int y, uint16_t* dstCParam,
-                             int count) SK_OVERRIDE;
+
+    virtual SkShader::Context* createContext(const SkBitmap&, const SkPaint&, const SkMatrix&,
+                                             void* storage) const SK_OVERRIDE;
+    virtual size_t contextSize() const SK_OVERRIDE;
+
+    class RadialGradientContext : public SkGradientShaderBase::GradientShaderBaseContext {
+    public:
+        RadialGradientContext(const SkRadialGradient& shader, const SkBitmap& device,
+                              const SkPaint& paint, const SkMatrix& matrix);
+        ~RadialGradientContext() {}
+
+        virtual void shadeSpan(int x, int y, SkPMColor dstC[], int count) SK_OVERRIDE;
+        virtual void shadeSpan16(int x, int y, uint16_t dstC[], int count) SK_OVERRIDE;
+
+    private:
+        typedef SkGradientShaderBase::GradientShaderBaseContext INHERITED;
+    };
+
     virtual BitmapType asABitmap(SkBitmap* bitmap,
                                  SkMatrix* matrix,
                                  TileMode* xy) const SK_OVERRIDE;