Introduce OnceClosure and BindOnce

base/callback_internal.cc

Index: base/callback_internal.cc
diff --git a/base/callback_internal.cc b/base/callback_internal.cc
index 4c8ccae932df1d2e9239903d67fae1745e5b7c1f..d4bb24c242e354803dce67b58b624dfca8aad78d 100644
--- a/base/callback_internal.cc
+++ b/base/callback_internal.cc
@@ -32,6 +32,17 @@ CallbackBase<CopyMode::MoveOnly>::operator=(CallbackBase&& c) {
   return *this;
 }
 
+CallbackBase<CopyMode::MoveOnly>::CallbackBase(
+    const CallbackBase<CopyMode::Copyable>& c)
+    : bind_state_(c.bind_state_), polymorphic_invoke_(c.polymorphic_invoke_) {}
+
+CallbackBase<CopyMode::MoveOnly>& CallbackBase<CopyMode::MoveOnly>::operator=(
+    const CallbackBase<CopyMode::Copyable>& c) {
+  bind_state_ = c.bind_state_;
+  polymorphic_invoke_ = c.polymorphic_invoke_;
+  return *this;
+}
+
 void CallbackBase<CopyMode::MoveOnly>::Reset() {
   polymorphic_invoke_ = nullptr;
   // NULL the bind_state_ last, since it may be holding the last ref to whatever