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

include/core/SkEmptyShader.h

Index: include/core/SkEmptyShader.h
diff --git a/include/core/SkEmptyShader.h b/include/core/SkEmptyShader.h
index d2ebb61221fed2f5425ad7168953a8b8a10d3cc3..7494eff3d079e0eee6d3c50f4adb300cbfcf663c 100644
--- a/include/core/SkEmptyShader.h
+++ b/include/core/SkEmptyShader.h
@@ -15,20 +15,28 @@
 
 /**
  *  \class SkEmptyShader
- *  A Shader that always draws nothing. Its setContext always returns false,
- *  so it never expects that its shadeSpan() methods will get called.
+ *  A Shader that always draws nothing. Its createContext always returns NULL.
  */
 class SK_API SkEmptyShader : public SkShader {
 public:
     SkEmptyShader() {}
 
-    virtual uint32_t getFlags() SK_OVERRIDE;
-    virtual uint8_t getSpan16Alpha() const SK_OVERRIDE;
-    virtual bool setContext(const SkBitmap&, const SkPaint&,
-                            const SkMatrix&) SK_OVERRIDE;
-    virtual void shadeSpan(int x, int y, SkPMColor span[], int count) SK_OVERRIDE;
-    virtual void shadeSpan16(int x, int y, uint16_t span[], int count) SK_OVERRIDE;
-    virtual void shadeSpanAlpha(int x, int y, uint8_t alpha[], int count) SK_OVERRIDE;
+    virtual size_t contextSize() const SK_OVERRIDE {
+        // Even though createContext returns NULL we have to return a value of at least
+        // sizeof(SkShader::Context) to satisfy SkSmallAllocator.
+        return sizeof(SkShader::Context);
+    }
+
+    virtual bool validContext(const SkBitmap&, const SkPaint&,
+                              const SkMatrix&, SkMatrix* totalInverse = NULL) const SK_OVERRIDE {
+        return false;
+    }
+
+    virtual SkShader::Context* createContext(const SkBitmap&, const SkPaint&,
+                                             const SkMatrix&, void*) const SK_OVERRIDE {
+        // validContext returns false.
+        return NULL;
+    }
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkEmptyShader)