tcmalloc: Use C++11 atomics where appropriate.

base/allocator/BUILD.gn

Index: base/allocator/BUILD.gn
diff --git a/base/allocator/BUILD.gn b/base/allocator/BUILD.gn
index ac534817fc8eb4252bbb2bfee5babd1bcd262e45..c86a7583c9cadb9faebd6727780666c38d50b8f8 100644
--- a/base/allocator/BUILD.gn
+++ b/base/allocator/BUILD.gn
@@ -122,14 +122,11 @@ if (use_allocator == "tcmalloc") {
       "$tcmalloc_dir/src/base/abort.cc",
       "$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/atomicops_internals_portable.h",
+      "$tcmalloc_dir/src/base/basictypes.h",
       "$tcmalloc_dir/src/base/commandlineflags.h",
       "$tcmalloc_dir/src/base/cycleclock.h",