Clear background to white in animatedGif GM

gm/animatedGif.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkAnimTimer.h"
 #include "SkCanvas.h"
@@ skipping 80 matching lines @@
             // Wide enough to display all the frames.
             dim.fWidth *= fTotalFrames;
             // Tall enough to show the row of frames plus an animating version.
             dim.fHeight *= 2;
             return dim;
         }
         return SkISize::Make(640, 480);
     }
 
     void onDrawBackground(SkCanvas* canvas) override {
+        canvas->clear(SK_ColorWHITE);
         if (this->initCodec()) {
             SkAutoCanvasRestore acr(canvas, true);
             for (size_t frameIndex = 0; frameIndex < fTotalFrames; frameIndex++) {
                 this->drawFrame(canvas, frameIndex);
                 canvas->translate(fCodec->getInfo().width(), 0);
             }
         }
     }
 
     bool initCodec() {
@@ skipping 65 matching lines @@
             // in order to draw correctly.
             fNextUpdate += fFrameInfos[fFrame].fDuration;
         }
 
         return true;
     }
 };
 
 DEF_GM(return new AnimatedGifGM);