Revert of Remove old/unused atomicops code

base/allocator/BUILD.gn

Index: base/allocator/BUILD.gn
diff --git a/base/allocator/BUILD.gn b/base/allocator/BUILD.gn
index c42de1a01a38900bdbf8e23029ffeb9b78441c61..a07a356e5d85a204d5f4bd3e3850f50b0e3ff43c 100644
--- a/base/allocator/BUILD.gn
+++ b/base/allocator/BUILD.gn
@@ -63,17 +63,6 @@
       "$tcmalloc_dir/src/base/abort.cc",
       "$tcmalloc_dir/src/base/abort.h",
       "$tcmalloc_dir/src/base/arm_instruction_set_select.h",
-      "$tcmalloc_dir/src/base/atomicops-internals-arm-generic.h",
-      "$tcmalloc_dir/src/base/atomicops-internals-arm-v6plus.h",
-      "$tcmalloc_dir/src/base/atomicops-internals-linuxppc.h",
-      "$tcmalloc_dir/src/base/atomicops-internals-macosx.h",
-      "$tcmalloc_dir/src/base/atomicops-internals-windows.h",
-      "$tcmalloc_dir/src/base/atomicops-internals-x86.cc",
-      "$tcmalloc_dir/src/base/atomicops-internals-x86.h",
-      "$tcmalloc_dir/src/base/atomicops.h",
-      "$tcmalloc_dir/src/base/basictypes.h",
-      "$tcmalloc_dir/src/base/commandlineflags.h",
-      "$tcmalloc_dir/src/base/cycleclock.h",
 
       # We don't list dynamic_annotations.c since its copy is already
       # present in the dynamic_annotations target.