Revert of Move WTF::SpinLock to base::SpinLock.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index cb6e3e9cf54d223f0a0387a5f108e0d62928b436..141dc2b7552a4bbbafa9824c1af0823ddd8a7910 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -1207,14 +1207,6 @@
       "rand_util_nacl.cc",
       "synchronization/read_write_lock_nacl.cc",
     ]
-
-    # Add stuff that doesn't work in NaCl.
-    sources += [
-      # SpinLock uses GNU-style inline assembly, which the pnacl-clang++
-      # compiler doesn't allow.
-      "synchronization/spin_lock.cc",
-      "synchronization/spin_lock.h",
-    ]
   }
 
   # Windows.