Move WTF::SpinLock to base::SpinLock.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 6e929147fa8e438b71da61d09e69392306979cfa..3059a19736fc350b00fca03cd49a62447344df47 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -19,6 +19,7 @@
 
 import("//build/buildflag_header.gni")
 import("//build/config/allocator.gni")
+import("//build/config/arm.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
@@ -1215,6 +1216,15 @@ component("base") {
     ]
   }
 
+  # 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 += [