Update SkMergeImageFilter to sk_sp

src/effects/SkDropShadowImageFilter.cpp

Index: src/effects/SkDropShadowImageFilter.cpp
diff --git a/src/effects/SkDropShadowImageFilter.cpp b/src/effects/SkDropShadowImageFilter.cpp
index 93c11ca1bb520d48aa673c8f67d55ce7d0edde21..6e750ca74c1b7f15a522a8527e8c4d0d2ef5ccbf 100644
--- a/src/effects/SkDropShadowImageFilter.cpp
+++ b/src/effects/SkDropShadowImageFilter.cpp
@@ -37,7 +37,7 @@ SkFlattenable* SkDropShadowImageFilter::CreateProc(SkReadBuffer& buffer) {
     ShadowMode shadowMode = buffer.isVersionLT(SkReadBuffer::kDropShadowMode_Version) ?
                             kDrawShadowAndForeground_ShadowMode :
                             static_cast<ShadowMode>(buffer.readInt());
-    return Create(dx, dy, sigmaX, sigmaY, color, shadowMode, common.getInput(0),
+    return Create(dx, dy, sigmaX, sigmaY, color, shadowMode, common.getInput(0).get(),
                   &common.cropRect());
 }