Revert of Revert[2] of add ImageShader, sharing code with its Bitmap cousin

src/image/SkImage_Gpu.h

Index: src/image/SkImage_Gpu.h
diff --git a/src/image/SkImage_Gpu.h b/src/image/SkImage_Gpu.h
index 8bfdfa7cd834e2fed6260f2dc64cb4f12d5fb952..5e847dd381cfe657225d67a71007dbd53d3b0a44 100644
--- a/src/image/SkImage_Gpu.h
+++ b/src/image/SkImage_Gpu.h
@@ -40,6 +40,9 @@
     SkImage* onNewSubset(const SkIRect&) const override;
 
     GrTexture* peekTexture() const override { return fTexture; }
+    SkShader* onNewShader(SkShader::TileMode,
+                          SkShader::TileMode,
+                          const SkMatrix* localMatrix) const override;
     bool isOpaque() const override;
     bool onReadPixels(const SkImageInfo&, void* dstPixels, size_t dstRowBytes,
                       int srcX, int srcY) const override;