Make Callback::Reset() return a copy to support use-cases where Run() ends up modifying |*this|.

base/callback.h

Index: base/callback.h
diff --git a/base/callback.h b/base/callback.h
index ab8f7bb8a711d39f85069b3fdcbf65a09948ba44..d8da740d21184d9133b6997d850f25490fdb6d00 100644
--- a/base/callback.h
+++ b/base/callback.h
@@ -39,6 +39,10 @@
 // The reason to pass via a const-reference is to avoid unnecessary
 // AddRef/Release pairs to the internal state.
 //
+// If a Callback variable must be empty during the Run() of a previous value of
+// that variable, use ResetAndRun().  E.g. if MyClass has a member variable
+// Callback which gets set as a side-effect of Run()'ing the Callback (to
+// something else), ResetAndRun() can be handy.
 //
 // EXAMPLE USAGE:
 //
@@ -272,6 +276,13 @@ class Callback<R(void)> : public internal::CallbackBase {
     return f(bind_state_.get());
   }
 
+
+  R ResetAndRun() {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run();
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*);
@@ -311,6 +322,13 @@ class Callback<R(A1)> : public internal::CallbackBase {
     return f(bind_state_.get(), internal::CallbackForward(a1));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,
@@ -353,6 +371,15 @@ class Callback<R(A1, A2)> : public internal::CallbackBase {
              internal::CallbackForward(a2));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1,
+        typename internal::CallbackParamTraits<A2>::ForwardType a2) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1),
+            internal::CallbackForward(a2));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,
@@ -398,6 +425,17 @@ class Callback<R(A1, A2, A3)> : public internal::CallbackBase {
              internal::CallbackForward(a3));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1,
+        typename internal::CallbackParamTraits<A2>::ForwardType a2,
+        typename internal::CallbackParamTraits<A3>::ForwardType a3) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1),
+            internal::CallbackForward(a2),
+            internal::CallbackForward(a3));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,
@@ -446,6 +484,19 @@ class Callback<R(A1, A2, A3, A4)> : public internal::CallbackBase {
              internal::CallbackForward(a4));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1,
+        typename internal::CallbackParamTraits<A2>::ForwardType a2,
+        typename internal::CallbackParamTraits<A3>::ForwardType a3,
+        typename internal::CallbackParamTraits<A4>::ForwardType a4) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1),
+            internal::CallbackForward(a2),
+            internal::CallbackForward(a3),
+            internal::CallbackForward(a4));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,
@@ -498,6 +549,21 @@ class Callback<R(A1, A2, A3, A4, A5)> : public internal::CallbackBase {
              internal::CallbackForward(a5));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1,
+        typename internal::CallbackParamTraits<A2>::ForwardType a2,
+        typename internal::CallbackParamTraits<A3>::ForwardType a3,
+        typename internal::CallbackParamTraits<A4>::ForwardType a4,
+        typename internal::CallbackParamTraits<A5>::ForwardType a5) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1),
+            internal::CallbackForward(a2),
+            internal::CallbackForward(a3),
+            internal::CallbackForward(a4),
+            internal::CallbackForward(a5));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,
@@ -553,6 +619,23 @@ class Callback<R(A1, A2, A3, A4, A5, A6)> : public internal::CallbackBase {
              internal::CallbackForward(a6));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1,
+        typename internal::CallbackParamTraits<A2>::ForwardType a2,
+        typename internal::CallbackParamTraits<A3>::ForwardType a3,
+        typename internal::CallbackParamTraits<A4>::ForwardType a4,
+        typename internal::CallbackParamTraits<A5>::ForwardType a5,
+        typename internal::CallbackParamTraits<A6>::ForwardType a6) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1),
+            internal::CallbackForward(a2),
+            internal::CallbackForward(a3),
+            internal::CallbackForward(a4),
+            internal::CallbackForward(a5),
+            internal::CallbackForward(a6));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,
@@ -611,6 +694,25 @@ class Callback<R(A1, A2, A3, A4, A5, A6, A7)> : public internal::CallbackBase {
              internal::CallbackForward(a7));
   }
 
+
+  R ResetAndRun(typename internal::CallbackParamTraits<A1>::ForwardType a1,
+        typename internal::CallbackParamTraits<A2>::ForwardType a2,
+        typename internal::CallbackParamTraits<A3>::ForwardType a3,
+        typename internal::CallbackParamTraits<A4>::ForwardType a4,
+        typename internal::CallbackParamTraits<A5>::ForwardType a5,
+        typename internal::CallbackParamTraits<A6>::ForwardType a6,
+        typename internal::CallbackParamTraits<A7>::ForwardType a7) {
+    Callback<RunType> tmp(*this);
+    Reset();
+    tmp.Run(internal::CallbackForward(a1),
+            internal::CallbackForward(a2),
+            internal::CallbackForward(a3),
+            internal::CallbackForward(a4),
+            internal::CallbackForward(a5),
+            internal::CallbackForward(a6),
+            internal::CallbackForward(a7));
+  }
+
  private:
   typedef R(*PolymorphicInvoke)(
       internal::BindStateBase*,