clang/win: Let some chromium_code targets build with -Wextra.

base/threading/thread_local_storage.cc

Index: base/threading/thread_local_storage.cc
diff --git a/base/threading/thread_local_storage.cc b/base/threading/thread_local_storage.cc
index 701f6a2af9bf5afd1ae12c4b89fc387963ced92d..ffa236f0bd8310921924489066f42385df22a416 100644
--- a/base/threading/thread_local_storage.cc
+++ b/base/threading/thread_local_storage.cc
@@ -76,8 +76,10 @@ void** ConstructTlsVector() {
     // TLS_KEY_OUT_OF_INDEXES, go ahead and set it.  Otherwise, do nothing, as
     // another thread already did our dirty work.
     if (PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES !=
-        base::subtle::NoBarrier_CompareAndSwap(&g_native_tls_key,
-            PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES, key)) {
+        static_cast<PlatformThreadLocalStorage::TLSKey>(
+            base::subtle::NoBarrier_CompareAndSwap(
+                &g_native_tls_key,
+                PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES, key))) {
       // We've been shortcut. Another thread replaced g_native_tls_key first so
       // we need to destroy our index and use the one the other thread got
       // first.