Extend base::Callback to have a move-only variant

base/callback_forward.h

Index: base/callback_forward.h
diff --git a/base/callback_forward.h b/base/callback_forward.h
index a9a263a50ea77517e6514792843660d6d25435ca..8b9b89cdc231bcf1020111e91bcbda14fd4e678c 100644
--- a/base/callback_forward.h
+++ b/base/callback_forward.h
@@ -6,8 +6,19 @@
 #define BASE_CALLBACK_FORWARD_H_
 
 namespace base {
+namespace internal {
 
-template <typename Sig>
+// CopyMode is used to control the copyablity of a Callback.
+// MoveOnly indicates the Callback is not copyable but movable, and Copyable
+// indicates it is copyable and movable.
+enum class CopyMode {
+  MoveOnly, Copyable,
+};
+
+}  // namespace internal
+
+template <typename Signature,
+          internal::CopyMode copy_mode = internal::CopyMode::Copyable>
 class Callback;
 
 // Syntactic sugar to make Callback<void()> easier to declare since it