Out of line CallbackBase's copy and assignment operators.

base/callback_internal.cc

Index: base/callback_internal.cc
diff --git a/base/callback_internal.cc b/base/callback_internal.cc
index ed0fb0dd0cce05e1566401726d063b6a9d25cb33..f360388273dac06d9dfe4eda1a9cc737c19ee7f9 100644
--- a/base/callback_internal.cc
+++ b/base/callback_internal.cc
@@ -9,6 +9,9 @@
 namespace base {
 namespace internal {
 
+CallbackBase::CallbackBase(const CallbackBase& c) = default;
+CallbackBase& CallbackBase::operator=(const CallbackBase& c) = default;
+
 void CallbackBase::Reset() {
   polymorphic_invoke_ = NULL;
   // NULL the bind_state_ last, since it may be holding the last ref to whatever