[Android] Fix compilation error: unused variable 'kAllowedToAccessOnNonjoinableThread'

content/child/child_thread.cc

Index: content/child/child_thread.cc
diff --git a/content/child/child_thread.cc b/content/child/child_thread.cc
index e38e3a5856e224eee4febc43777728d909a92000..8c160ada60eb15660d340a3bb9cf56a978d639e2 100644
--- a/content/child/child_thread.cc
+++ b/content/child/child_thread.cc
@@ -166,7 +166,10 @@ base::LazyInstance<base::Lock> g_lazy_child_thread_lock =
 // doesn't handle the case. Thus, we need our own class here.
 struct CondVarLazyInstanceTraits {
   static const bool kRegisterOnExit = true;
-  static const bool kAllowedToAccessOnNonjoinableThread ALLOW_UNUSED = false;
+#ifndef NDEBUG
+  static const bool kAllowedToAccessOnNonjoinableThread = false;
+#endif
+
   static base::ConditionVariable* New(void* instance) {
     return new (instance) base::ConditionVariable(
         g_lazy_child_thread_lock.Pointer());