Expose how many bytes are written in ProtoOutputStream

Bug: 69428467
Test: N/A
Change-Id: If1405885949635cebd1c6d89e220c210ef9fa210
diff --git a/libs/protoutil/src/ProtoOutputStream.cpp b/libs/protoutil/src/ProtoOutputStream.cpp
index 9d8ee729..1904d40 100644
--- a/libs/protoutil/src/ProtoOutputStream.cpp
+++ b/libs/protoutil/src/ProtoOutputStream.cpp
@@ -295,6 +295,12 @@
     }
 }
 
+size_t
+ProtoOutputStream::bytesWritten()
+{
+    return mBuffer.size();
+}
+
 bool
 ProtoOutputStream::compact() {
     if (mCompact) return true;