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 737b30d1db6e3736dcede3736d2bb23bc0bf09d0..fbcac10e89c1fe868bb4e42a76c7a957d7156ca8 100644
--- a/src/image/SkImage_Base.h
+++ b/src/image/SkImage_Base.h
@@ -61,10 +61,6 @@ public:
     // Caller must call unref when they are done.
     virtual GrTexture* asTextureRef(GrContext*, SkImageUsageType) const = 0;
 
-    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;