spin off easy stuff from Herb's windows GN CL

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2209533004

No public API changes.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2209533004
diff --git a/gm/shadows.cpp b/gm/shadows.cpp
index 6263af2..fcaeeb7 100644
--- a/gm/shadows.cpp
+++ b/gm/shadows.cpp
@@ -8,6 +8,8 @@
 #include "gm.h"
 #include "SkBlurDrawLooper.h"
 #include "SkBlurMask.h"
+#include "SkColorFilter.h"
+#include "SkMaskFilter.h"
 #include "SkPath.h"
 
 namespace skiagm {