Mark base::Callback default constructor as constexpr

Change-Id: If9fec539c2fc140d755033a1ed5dbaff141230d4
Reviewed-on: https://chromium-review.googlesource.com/974725Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545013}

base/callback.h

@@ -56,7 +56,7 @@ class OnceCallback<R(Args...)> : public internal::CallbackBase {
   using PolymorphicInvoke = R (*)(internal::BindStateBase*,
                                   internal::PassingTraitsType<Args>...);
 
-  OnceCallback() : internal::CallbackBase(nullptr) {}
+  constexpr OnceCallback() = default;
 
   explicit OnceCallback(internal::BindStateBase* bind_state)
       : internal::CallbackBase(bind_state) {}
@@ -103,7 +103,7 @@ class RepeatingCallback<R(Args...)> : public internal::CallbackBaseCopyable {
   using PolymorphicInvoke = R (*)(internal::BindStateBase*,
                                   internal::PassingTraitsType<Args>...);
 
-  RepeatingCallback() : internal::CallbackBaseCopyable(nullptr) {}
+  constexpr RepeatingCallback() = default;
 
   explicit RepeatingCallback(internal::BindStateBase* bind_state)
       : internal::CallbackBaseCopyable(bind_state) {}

base/callback_internal.h

@@ -134,6 +134,8 @@ class BASE_EXPORT CallbackBase {
   // Returns true if this callback equals |other|. |other| may be null.
   bool EqualsInternal(const CallbackBase& other) const;
 
+  constexpr inline CallbackBase();
+
   // Allow initializing of |bind_state_| via the constructor to avoid default
   // initialization of the scoped_refptr.
   explicit CallbackBase(BindStateBase* bind_state);
@@ -150,6 +152,8 @@ class BASE_EXPORT CallbackBase {
   scoped_refptr<BindStateBase> bind_state_;
 };
 
+constexpr CallbackBase::CallbackBase() = default;
+
 // CallbackBase<Copyable> is a direct base class of Copyable Callbacks.
 class BASE_EXPORT CallbackBaseCopyable : public CallbackBase {
  public:
@@ -159,6 +163,7 @@ class BASE_EXPORT CallbackBaseCopyable : public CallbackBase {
   CallbackBaseCopyable& operator=(CallbackBaseCopyable&& c);
 
  protected:
+  constexpr CallbackBaseCopyable() = default;
   explicit CallbackBaseCopyable(BindStateBase* bind_state)
       : CallbackBase(bind_state) {}
   ~CallbackBaseCopyable() = default;