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

gm/matriximagefilter.cpp

Index: gm/matriximagefilter.cpp
diff --git a/gm/matriximagefilter.cpp b/gm/matriximagefilter.cpp
index a42870b617ea7110b292b40330755022a718f7bd..419c4199dee1864eebee185eca569a22094ed989 100644
--- a/gm/matriximagefilter.cpp
+++ b/gm/matriximagefilter.cpp
@@ -58,7 +58,7 @@ protected:
     }
 
     virtual void onDraw(SkCanvas* canvas) {
-        canvas->clear(0x00000000);
+        canvas->clear(SK_ColorBLACK);
         SkMatrix matrix;
         SkScalar margin = SkIntToScalar(10);
         matrix.setSkew(SkDoubleToScalar(0.5), SkDoubleToScalar(0.2));