Fix alpha computation in SkArithmeticMode.

Note: this will require new baselines for the arithmode GM.

R=reed@google.com

Review URL: https://codereview.chromium.org/15917010

git-svn-id: http://skia.googlecode.com/svn/trunk@9302 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/arithmode.cpp b/gm/arithmode.cpp
index f5e1091..2a97eca 100644
--- a/gm/arithmode.cpp
+++ b/gm/arithmode.cpp
@@ -29,8 +29,8 @@
     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);