fix MSVC warning 4245: conversion signed => unsigned during initialization (#678)

absl/container/internal/container_memory.h

diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index 3487ac1..536ea39 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -250,8 +250,8 @@
 // type, which is non-portable.
 template <class Pair, class = std::true_type>
 struct OffsetOf {
-  static constexpr size_t kFirst = -1;
-  static constexpr size_t kSecond = -1;
+  static constexpr size_t kFirst = static_cast<size_t>(-1);
+  static constexpr size_t kSecond = static_cast<size_t>(-1);
 };
 
 template <class Pair>