Move texture drawing utility method to SkGpuDevice

BUG=skia:4542

Review URL: https://codereview.chromium.org/1506203002
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 5bf074d..219b765 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -140,6 +140,9 @@
 
     static SkImageFilter::Cache* NewImageFilterCache();
 
+    // for debugging purposes only
+    void drawTexture(GrTexture*, const SkRect& dst, const SkPaint&);
+
 protected:
     bool onReadPixels(const SkImageInfo&, void*, size_t, int, int) override;
     bool onWritePixels(const SkImageInfo&, const void*, size_t, int, int) override;