Constify SkFlattenable::getFactory().

src/core/SkFlattenableBuffers.cpp

Index: src/core/SkFlattenableBuffers.cpp
diff --git a/src/core/SkFlattenableBuffers.cpp b/src/core/SkFlattenableBuffers.cpp
index 50a47d5c47c209857f468fae86a575f0bd7c8de2..5167c2ebe9067b1a41c0b3785f0ef37f2cb7ac81 100644
--- a/src/core/SkFlattenableBuffers.cpp
+++ b/src/core/SkFlattenableBuffers.cpp
@@ -51,6 +51,7 @@ void SkFlattenableWriteBuffer::writePaint(const SkPaint& paint) {
     paint.flatten(*this);
 }
 
-void SkFlattenableWriteBuffer::flattenObject(SkFlattenable* obj, SkFlattenableWriteBuffer& buffer) {
+void SkFlattenableWriteBuffer::flattenObject(const SkFlattenable* obj,
+                                             SkFlattenableWriteBuffer& buffer) {
     obj->flatten(buffer);
 }