Adding an option to render only the shadow in SkDropShadowImageFilter

gm/imagefilterscropexpand.cpp

Index: gm/imagefilterscropexpand.cpp
diff --git a/gm/imagefilterscropexpand.cpp b/gm/imagefilterscropexpand.cpp
index c9601b066c91eb8c74d6f6043d2757cf6e117a9d..4fe746a1a14ce2239276f09f332958512c5fcf24 100644
--- a/gm/imagefilterscropexpand.cpp
+++ b/gm/imagefilterscropexpand.cpp
@@ -142,7 +142,8 @@ protected:
 
             draw(canvas, checkerboard, rect, SkDropShadowImageFilter::Create(
                 SkIntToScalar(10), SkIntToScalar(10), SkIntToScalar(3), SkIntToScalar(3),
-                SK_ColorBLUE, noop_cropped.get(), &big_rect));
+                SK_ColorBLUE, SkDropShadowImageFilter::kDrawShadowAndForeground_ShadowMode,
+                noop_cropped.get(), &big_rect));
 
             draw(canvas, checkerboard, rect, SkDisplacementMapEffect::Create(
                 SkDisplacementMapEffect::kR_ChannelSelectorType,