add ImageShader, sharing code with its Bitmap cousin

src/image/SkImage_Base.h

Index: src/image/SkImage_Base.h
diff --git a/src/image/SkImage_Base.h b/src/image/SkImage_Base.h
index d31626764723bbf707aacc4f9e82f6235d612878..22f952ef7b748b95a06061faf1407a4a54ff695e 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -58,10 +58,6 @@ public:
     // but only inspect them (or encode them).
     virtual bool getROPixels(SkBitmap*) const { return false; }
 
-    virtual SkShader* onNewShader(SkShader::TileMode,
-                                  SkShader::TileMode,
-                                  const SkMatrix* localMatrix) const { return nullptr; }
-
     // newWidth > 0, newHeight > 0, subset either nullptr or a proper subset of this bounds
     virtual SkImage* onNewImage(int newWidth, int newHeight, const SkIRect* subset,
                                 SkFilterQuality) const;