Create a new API allowing callers to concat a serialized message field.

Test: N/A
Change-Id: I90570ea62e6c406d40c399cee6cbb8dcb5172e21
diff --git a/libs/protoutil/include/android/util/ProtoOutputStream.h b/libs/protoutil/include/android/util/ProtoOutputStream.h
index 10be649..b8415b2 100644
--- a/libs/protoutil/include/android/util/ProtoOutputStream.h
+++ b/libs/protoutil/include/android/util/ProtoOutputStream.h
@@ -66,7 +66,7 @@
     EncodedBuffer::iterator data();
     bool flush(int fd);
 
-    // Please don't use the following functions to dump protos unless you are sure about it.
+    // Please don't use the following functions to dump protos unless you are familiar with protobuf encoding.
     void writeRawVarint(uint64_t varint);
     void writeLengthDelimitedHeader(uint32_t id, size_t size);
     void writeRawByte(uint8_t byte);
@@ -94,6 +94,7 @@
     inline void writeEnumImpl(uint32_t id, int val);
     inline void writeBoolImpl(uint32_t id, bool val);
     inline void writeUtf8StringImpl(uint32_t id, const char* val, size_t size);
+    inline void writeMessageBytesImpl(uint32_t id, const char* val, size_t size);
 
     bool compact();
     size_t editEncodedSize(size_t rawSize);