Fix alpha computation in SkArithmeticMode.

gm/arithmode.cpp

Index: gm/arithmode.cpp
diff --git a/gm/arithmode.cpp b/gm/arithmode.cpp
index f5e1091effd6de542f2c909b0d42bff25cd4b456..2a97eca957e525762714d918faeb0f5fb1d6b0bd 100644
--- a/gm/arithmode.cpp
+++ b/gm/arithmode.cpp
@@ -29,8 +29,8 @@ static SkBitmap make_src() {
     SkPaint paint;
     SkPoint pts[] = { {0, 0}, {SkIntToScalar(WW), SkIntToScalar(HH)} };
     SkColor colors[] = {
-        SK_ColorBLACK, SK_ColorGREEN, SK_ColorCYAN,
-        SK_ColorRED, SK_ColorMAGENTA, SK_ColorWHITE
+        SK_ColorTRANSPARENT, SK_ColorGREEN, SK_ColorCYAN,
+        SK_ColorRED, SK_ColorMAGENTA, SK_ColorWHITE,
     };
     SkShader* s = SkGradientShader::CreateLinear(pts, colors, NULL, SK_ARRAY_COUNT(colors),
                                                  SkShader::kClamp_TileMode);