Adding an option to render only the shadow in SkDropShadowImageFilter

This is basically how blink uses the filter. Currently, I can't use it for "ShadowOnly" mode with the filter at all, but instead of copying the code and risking to have the codepaths diverge, I'm simply going to add the option here.

BUG=skia:

Review URL: https://codereview.chromium.org/646213004

1 files changed, 2 insertions, 1 deletions

diff --git a/gm/imagefiltersbase.cpp b/gm/imagefiltersbase.cpp
index d1bd5a72df..4546b8add2 100644
--- a/gm/imagefiltersbase.cpp
+++ b/gm/imagefiltersbase.cpp
@@ -231,7 +231,8 @@ protected:
             FailImageFilter::Create(),
             SkColorFilterImageFilter::Create(cf),
             SkBlurImageFilter::Create(12.0f, 0.0f),
-            SkDropShadowImageFilter::Create(10.0f, 5.0f, 3.0f, 3.0f, SK_ColorBLUE),
+            SkDropShadowImageFilter::Create(10.0f, 5.0f, 3.0f, 3.0f, SK_ColorBLUE,
+                SkDropShadowImageFilter::kDrawShadowAndForeground_ShadowMode),
         };
         cf->unref();