Replace SIZE_MAX with std::numeric_limits.

helpers/memenv/memenv.cc used SIZE_MAX without including <stdint.h>.
Since we're fixing this problem, replace SIZE_MAX with
std::numeric_limits<size_t>::max(), which is clearer.

Fixes https://github.com/google/leveldb/issues/562

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189821707

helpers/memenv/memenv.cc

@@ -6,6 +6,7 @@
 
 #include <string.h>
 
+#include <limits>
 #include <map>
 #include <string>
 #include <vector>
@@ -65,7 +66,7 @@ class FileState {
       return Status::OK();
     }
 
-    assert(offset / kBlockSize <= SIZE_MAX);
+    assert(offset / kBlockSize <= std::numeric_limits<size_t>::max());
     size_t block = static_cast<size_t>(offset / kBlockSize);
     size_t block_offset = offset % kBlockSize;