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

N.B.: this will change results on the bots for all the tests modified.

BUG=skia:3319

Review URL: https://codereview.chromium.org/1077763002
diff --git a/gm/xfermodeimagefilter.cpp b/gm/xfermodeimagefilter.cpp
index c764f1c..649e69f 100644
--- a/gm/xfermodeimagefilter.cpp
+++ b/gm/xfermodeimagefilter.cpp
@@ -74,7 +74,7 @@
     }
 
     void onDraw(SkCanvas* canvas) override {
-        canvas->clear(0x00000000);
+        canvas->clear(SK_ColorBLACK);
         SkPaint paint;
 
         const struct {