Introduce OnceClosure and BindOnce

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index 6de18fd39d505b42ef4c07c906ae2c4086bf59e8..989572afb117e60df02f34ac7e09f378731b8a0b 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -77,6 +77,9 @@ class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
   CallbackBase(CallbackBase&& c);
   CallbackBase& operator=(CallbackBase&& c);
 
+  explicit CallbackBase(const CallbackBase<CopyMode::Copyable>& c);
+  CallbackBase& operator=(const CallbackBase<CopyMode::Copyable>& c);
+
   // Returns true if Callback is null (doesn't refer to anything).
   bool is_null() const { return bind_state_.get() == NULL; }
   explicit operator bool() const { return !is_null(); }