add asAShadowBlur for android to drawlooper

src/core/SkMaskFilter.cpp

Index: src/core/SkMaskFilter.cpp
diff --git a/src/core/SkMaskFilter.cpp b/src/core/SkMaskFilter.cpp
index 9b023d0d65cb0e3ea40d24c35e2e6ff77bde360d..8b9792c2c4fbddf182acb3726586709a34a13693 100644
--- a/src/core/SkMaskFilter.cpp
+++ b/src/core/SkMaskFilter.cpp
@@ -26,6 +26,10 @@ bool SkMaskFilter::filterMask(SkMask*, const SkMask&, const SkMatrix&,
     return false;
 }
 
+bool SkMaskFilter::asABlur(BlurRec*) const {
+    return false;
+}
+
 static void extractMaskSubset(const SkMask& src, SkMask* dst) {
     SkASSERT(src.fBounds.contains(dst->fBounds));