Avoid copy on move-conversion from RepeatingCallback to OnceCallback

CallbackBase didn't have the move-conversion constructor and move-conversion
assignment. That causes an extra copy on the conversion from rvalue
RepeatingCallback to OnceCallback.

Review-Url: https://codereview.chromium.org/2740703002
Cr-Commit-Position: refs/heads/master@{#455404}

base/callback_internal.cc

@@ -54,6 +54,16 @@ CallbackBase<CopyMode::MoveOnly>& CallbackBase<CopyMode::MoveOnly>::operator=(
   return *this;
 }
 
+CallbackBase<CopyMode::MoveOnly>::CallbackBase(
+    CallbackBase<CopyMode::Copyable>&& c)
+    : bind_state_(std::move(c.bind_state_)) {}
+
+CallbackBase<CopyMode::MoveOnly>& CallbackBase<CopyMode::MoveOnly>::operator=(
+    CallbackBase<CopyMode::Copyable>&& c) {
+  bind_state_ = std::move(c.bind_state_);
+  return *this;
+}
+
 void CallbackBase<CopyMode::MoveOnly>::Reset() {
   // NULL the bind_state_ last, since it may be holding the last ref to whatever
   // object owns us, and we may be deleted after that.

base/callback_internal.h

@@ -82,6 +82,9 @@ class BASE_EXPORT CallbackBase<CopyMode::MoveOnly> {
   explicit CallbackBase(const CallbackBase<CopyMode::Copyable>& c);
   CallbackBase& operator=(const CallbackBase<CopyMode::Copyable>& c);
 
+  explicit CallbackBase(CallbackBase<CopyMode::Copyable>&& c);
+  CallbackBase& operator=(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(); }