Don't serialize anything for SkEmptyShader.

BUG=skia:3735

Review URL: https://codereview.chromium.org/1097433002

gm/badpaint.cpp

@@ -30,12 +30,10 @@ protected:
         SkMatrix badMatrix;
         badMatrix.setAll(0, 0, 0, 0, 0, 0, 0, 0, 0);
 
-#if 0 // This crashes pipe!
         // Empty bitmap.
         fPaints.push_back().setColor(SK_ColorGREEN);
         fPaints.back().setShader(SkShader::CreateBitmapShader(emptyBmp, SkShader::kClamp_TileMode,
                                                               SkShader::kClamp_TileMode))->unref();
-#endif
 
         // Non-invertible local matrix.
         fPaints.push_back().setColor(SK_ColorGREEN);

src/core/SkEmptyShader.h

@@ -34,6 +34,12 @@ protected:
         return NULL;
     }
 
+    void flatten(SkWriteBuffer& buffer) const override {
+        // Do nothing.
+        // We just don't want to fall through to SkShader::flatten(),
+        // which will write data we don't care to serialize or decode.
+    }
+
 private:
     typedef SkShader INHERITED;
 };