Move spin_lock into partition_allocator.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index f82f436086f9a6c357125d25b8d023b752749a62..995becb5af36f33ad6ae8853d7b90bd374710a35 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -1311,19 +1311,12 @@ component("base") {
         "allocator/partition_allocator/page_allocator.h",
         "allocator/partition_allocator/partition_alloc.cc",
         "allocator/partition_allocator/partition_alloc.h",
+        "allocator/partition_allocator/spin_lock.cc",
+        "allocator/partition_allocator/spin_lock.h",
       ]
     }
   }
 
-  # SpinLock uses inline assembly that doesn't work on NaCl, and for which there
-  # is no code for ARMv6.
-  if (!is_nacl && (current_cpu != "arm" || arm_version >= 7)) {
-    sources += [
-      "synchronization/spin_lock.cc",
-      "synchronization/spin_lock.h",
-    ]
-  }
-
   # Windows.
   if (is_win) {
     sources += [