add ImageShader, sharing code with its Bitmap cousin

src/image/SkImage_Gpu.cpp

Index: src/image/SkImage_Gpu.cpp
diff --git a/src/image/SkImage_Gpu.cpp b/src/image/SkImage_Gpu.cpp
index 163e62e02df540362952e9b55be47d3284d886a7..527bcc3ef558924d7a7aa753d4e62fea1e390eb2 100644
--- a/src/image/SkImage_Gpu.cpp
+++ b/src/image/SkImage_Gpu.cpp
@@ -49,13 +49,6 @@ extern void SkTextureImageApplyBudgetedDecision(SkImage* image) {
     }
 }
 
-SkShader* SkImage_Gpu::onNewShader(SkShader::TileMode tileX, SkShader::TileMode tileY,
-                                   const SkMatrix* localMatrix) const {
-    SkBitmap bm;
-    GrWrapTextureInBitmap(fTexture, this->width(), this->height(), this->isOpaque(), &bm);
-    return SkShader::CreateBitmapShader(bm, tileX, tileY, localMatrix);
-}
-
 bool SkImage_Gpu::getROPixels(SkBitmap* dst) const {
     if (SkBitmapCache::Find(this->uniqueID(), dst)) {
         SkASSERT(dst->getGenerationID() == this->uniqueID());