Enable flattening/unflattening with custom unflatten procs

src/core/SkValidatingReadBuffer.h

Index: src/core/SkValidatingReadBuffer.h
diff --git a/src/core/SkValidatingReadBuffer.h b/src/core/SkValidatingReadBuffer.h
index 7fb203b45e99d8a6755b06aa936a09752a10ee54..05bfe79a32c4413527c789edd4ef38e8ac6b9ea1 100644
--- a/src/core/SkValidatingReadBuffer.h
+++ b/src/core/SkValidatingReadBuffer.h
@@ -33,6 +33,9 @@ public:
     uint32_t readUInt() override;
     int32_t read32() override;
 
+    // peek
+    uint8_t peekByte() override;
+
     // strings -- the caller is responsible for freeing the string contents
     void readString(SkString* string) override;
     void* readEncodedString(size_t* length, SkPaint::TextEncoding encoding) override;