Change some more GMs to clear to opaque black, not transparent black.

gm/perlinnoise.cpp

Index: gm/perlinnoise.cpp
diff --git a/gm/perlinnoise.cpp b/gm/perlinnoise.cpp
index b69b7d0476a5b0a8ecdab57d9b56b748e57e816b..702e545c60268b5ef25bb31a80948bf1663cc995 100644
--- a/gm/perlinnoise.cpp
+++ b/gm/perlinnoise.cpp
@@ -58,7 +58,7 @@ protected:
     }
 
     virtual void onDraw(SkCanvas* canvas) {
-        canvas->clear(0x00000000);
+        canvas->clear(SK_ColorBLACK);
         test(canvas,   0,   0, SkPerlinNoiseShader::kFractalNoise_Type,
              0.1f, 0.1f, 0, 0, false);
         test(canvas, 100,   0, SkPerlinNoiseShader::kTurbulence_Type,