Simplify flattening to just write enough to call the factory

src/effects/SkMatrixImageFilter.cpp

Index: src/effects/SkMatrixImageFilter.cpp
diff --git a/src/effects/SkMatrixImageFilter.cpp b/src/effects/SkMatrixImageFilter.cpp
index c7c815a761dec37b3acb782bba71220ee4a69352..1bc47d3a114e6380e8b760e6bfd3ea782a617acf 100644
--- a/src/effects/SkMatrixImageFilter.cpp
+++ b/src/effects/SkMatrixImageFilter.cpp
@@ -29,11 +29,21 @@ SkMatrixImageFilter* SkMatrixImageFilter::Create(const SkMatrix& transform,
     return SkNEW_ARGS(SkMatrixImageFilter, (transform, filterLevel, input));
 }
 
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
 SkMatrixImageFilter::SkMatrixImageFilter(SkReadBuffer& buffer)
   : INHERITED(1, buffer) {
     buffer.readMatrix(&fTransform);
     fFilterLevel = static_cast<SkPaint::FilterLevel>(buffer.readInt());
 }
+#endif
+
+SkFlattenable* SkMatrixImageFilter::CreateProc(SkReadBuffer& buffer) {
+    SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 1);
+    SkMatrix matrix;
+    buffer.readMatrix(&matrix);
+    SkPaint::FilterLevel level = static_cast<SkPaint::FilterLevel>(buffer.readInt());
+    return Create(matrix, level, common.inputAt(0));
+}
 
 void SkMatrixImageFilter::flatten(SkWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);