Update from https://crrev.com/312398

base/atomicops.h

Index: base/atomicops.h
diff --git a/base/atomicops.h b/base/atomicops.h
index 833e1704291b1d55842e985a4d8dc7c2948d9520..6a5371c75c295e87d33e21fc16ddc89d4937e4d5 100644
--- a/base/atomicops.h
+++ b/base/atomicops.h
@@ -28,10 +28,14 @@
 #ifndef BASE_ATOMICOPS_H_
 #define BASE_ATOMICOPS_H_
 
-#include <cassert>  // Small C++ header which defines implementation specific
-                    // macros used to identify the STL implementation.
 #include <stdint.h>
 
+// Small C++ header which defines implementation specific macros used to
+// identify the STL implementation.
+// - libc++: captures __config for _LIBCPP_VERSION
+// - libstdc++: captures bits/c++config.h for __GLIBCXX__
+#include <cstddef>
+
 #include "base/base_export.h"
 #include "build/build_config.h"