Clear background to white in animatedGif GM

gm/animatedGif.cpp

Index: gm/animatedGif.cpp
diff --git a/gm/animatedGif.cpp b/gm/animatedGif.cpp
index fc6fad08868c6a654066919254487fd790d3d49c..6f7f0ddd1b2fe02885240e091a52a704813d47c5 100644
--- a/gm/animatedGif.cpp
+++ b/gm/animatedGif.cpp
@@ -98,6 +98,7 @@ private:
     }
 
     void onDrawBackground(SkCanvas* canvas) override {
+        canvas->clear(SK_ColorWHITE);
         if (this->initCodec()) {
             SkAutoCanvasRestore acr(canvas, true);
             for (size_t frameIndex = 0; frameIndex < fTotalFrames; frameIndex++) {