Fix unused variable warning in the Win-Clang build

base/allocator/allocator_shim.cc

Index: base/allocator/allocator_shim.cc
diff --git a/base/allocator/allocator_shim.cc b/base/allocator/allocator_shim.cc
index 57a398aaa55e089de33f119912c5ce7d11558de8..95480ea4b6b510f52a244d529da7a16385c9d035 100644
--- a/base/allocator/allocator_shim.cc
+++ b/base/allocator/allocator_shim.cc
@@ -34,7 +34,10 @@ subtle::AtomicWord g_chain_head = reinterpret_cast<subtle::AtomicWord>(
     &allocator::AllocatorDispatch::default_dispatch);
 
 bool g_call_new_handler_on_malloc_failure = false;
+
+#if !defined(OS_WIN)
 subtle::Atomic32 g_new_handler_lock = 0;
+#endif
 
 // In theory this should be just base::ThreadChecker. But we can't afford
 // the luxury of a LazyInstance<ThreadChecker> here as it would cause a new().