Revert of Remove old/unused atomicops code

base/atomicops.h

Index: base/atomicops.h
diff --git a/base/atomicops.h b/base/atomicops.h
index f983b455f193d2227b72c42e10caa8ba18708772..6a5371c75c295e87d33e21fc16ddc89d4937e4d5 100644
--- a/base/atomicops.h
+++ b/base/atomicops.h
@@ -144,6 +144,33 @@
 }  // namespace subtle
 }  // namespace base
 
+// The following x86 CPU features are used in atomicops_internals_x86_gcc.h, but
+// this file is duplicated inside of Chrome: protobuf and tcmalloc rely on the
+// struct being present at link time. Some parts of Chrome can currently use the
+// portable interface whereas others still use GCC one. The include guards are
+// the same as in atomicops_internals_x86_gcc.cc.
+#if defined(__i386__) || defined(__x86_64__)
+// This struct is not part of the public API of this module; clients may not
+// use it.  (However, it's exported via BASE_EXPORT because clients implicitly
+// do use it at link time by inlining these functions.)
+// Features of this x86.  Values may not be correct before main() is run,
+// but are set conservatively.
+struct AtomicOps_x86CPUFeatureStruct {
+  bool has_amd_lock_mb_bug; // Processor has AMD memory-barrier bug; do lfence
+                            // after acquire compare-and-swap.
+  // The following fields are unused by Chrome's base implementation but are
+  // still used by copies of the same code in other parts of the code base. This
+  // causes an ODR violation, and the other code is likely reading invalid
+  // memory.
+  // TODO(jfb) Delete these fields once the rest of the Chrome code base doesn't
+  //           depend on them.
+  bool has_sse2;            // Processor has SSE2.
+  bool has_cmpxchg16b;      // Processor supports cmpxchg16b instruction.
+};
+BASE_EXPORT extern struct AtomicOps_x86CPUFeatureStruct
+    AtomicOps_Internalx86CPUFeatures;
+#endif
+
 // Try to use a portable implementation based on C++11 atomics.
 //
 // Some toolchains support C++11 language features without supporting library
@@ -161,6 +188,17 @@
 #    include "base/atomicops_internals_x86_msvc.h"
 #  elif defined(OS_MACOSX)
 #    include "base/atomicops_internals_mac.h"
+#  elif defined(OS_NACL)
+#    include "base/atomicops_internals_gcc.h"
+#  elif defined(COMPILER_GCC) && defined(ARCH_CPU_ARMEL)
+#    include "base/atomicops_internals_arm_gcc.h"
+#  elif defined(COMPILER_GCC) && defined(ARCH_CPU_ARM64)
+#    include "base/atomicops_internals_arm64_gcc.h"
+#  elif defined(COMPILER_GCC) && defined(ARCH_CPU_X86_FAMILY)
+#    include "base/atomicops_internals_x86_gcc.h"
+#  elif (defined(COMPILER_GCC) && \
+         (defined(ARCH_CPU_MIPS_FAMILY) || defined(ARCH_CPU_MIPS64_FAMILY)))
+#    include "base/atomicops_internals_mips_gcc.h"
 #  else
 #    error "Atomic operations are not supported on your platform"
 #  endif