clear bitmap in texture domain gm

R=robertphillips@google.com
TBR=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/227563007

git-svn-id: http://skia.googlecode.com/svn/trunk@14082 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/texturedomaineffect.cpp b/gm/texturedomaineffect.cpp
index 3643369..babf800 100644
--- a/gm/texturedomaineffect.cpp
+++ b/gm/texturedomaineffect.cpp
@@ -46,6 +46,7 @@
     virtual void onOnceBeforeDraw() SK_OVERRIDE {
         fBmp.allocN32Pixels(100, 100);
         SkCanvas canvas(fBmp);
+        canvas.clear(0x00000000);
         SkPaint paint;
 
         SkColor colors1[] = { SK_ColorCYAN, SK_ColorLTGRAY, SK_ColorGRAY };