leveldb: Make WriteBatch::ApproximateSize() const.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229395810

db/write_batch.cc

@@ -39,7 +39,7 @@ void WriteBatch::Clear() {
   rep_.resize(kHeader);
 }
 
-size_t WriteBatch::ApproximateSize() {
+size_t WriteBatch::ApproximateSize() const {
   return rep_.size();
 }
 

include/leveldb/write_batch.h

@@ -52,7 +52,7 @@ class LEVELDB_EXPORT WriteBatch {
   //
   // This number is tied to implementation details, and may change across
   // releases. It is intended for LevelDB usage metrics.
-  size_t ApproximateSize();
+  size_t ApproximateSize() const;
 
   // Copies the operations in "source" to this batch.
   //