Remove `Runnable` concept from base/bind_internal.h

base/bind_helpers.h

Index: base/bind_helpers.h
diff --git a/base/bind_helpers.h b/base/bind_helpers.h
index 29057baef20cb000b91be992c605cfba18d2834f..a7c2acc09c9703a1e07a8a274f3dc53deb53d428 100644
--- a/base/bind_helpers.h
+++ b/base/bind_helpers.h
@@ -176,86 +176,6 @@ struct IsWeakReceiver;
 
 namespace internal {
 
-// Use the Substitution Failure Is Not An Error (SFINAE) trick to inspect T
-// for the existence of AddRef() and Release() functions of the correct
-// signature.
-//
-// http://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error
-// http://stackoverflow.com/questions/257288/is-it-possible-to-write-a-c-template-to-check-for-a-functions-existence
-// http://stackoverflow.com/questions/4358584/sfinae-approach-comparison
-// http://stackoverflow.com/questions/1966362/sfinae-to-check-for-inherited-member-functions
-//
-// The last link in particular show the method used below.
-//
-// For SFINAE to work with inherited methods, we need to pull some extra tricks
-// with multiple inheritance.  In the more standard formulation, the overloads
-// of Check would be:
-//
-//   template <typename C>
-//   Yes NotTheCheckWeWant(Helper<&C::TargetFunc>*);
-//
-//   template <typename C>
-//   No NotTheCheckWeWant(...);
-//
-//   static const bool value = sizeof(NotTheCheckWeWant<T>(0)) == sizeof(Yes);
-//
-// The problem here is that template resolution will not match
-// C::TargetFunc if TargetFunc does not exist directly in C.  That is, if
-// TargetFunc in inherited from an ancestor, &C::TargetFunc will not match,
-// |value| will be false.  This formulation only checks for whether or
-// not TargetFunc exist directly in the class being introspected.
-//
-// To get around this, we play a dirty trick with multiple inheritance.
-// First, We create a class BaseMixin that declares each function that we
-// want to probe for.  Then we create a class Base that inherits from both T
-// (the class we wish to probe) and BaseMixin.  Note that the function
-// signature in BaseMixin does not need to match the signature of the function
-// we are probing for; thus it's easiest to just use void().
-//
-// Now, if TargetFunc exists somewhere in T, then &Base::TargetFunc has an
-// ambiguous resolution between BaseMixin and T.  This lets us write the
-// following:
-//
-//   template <typename C>
-//   No GoodCheck(Helper<&C::TargetFunc>*);
-//
-//   template <typename C>
-//   Yes GoodCheck(...);
-//
-//   static const bool value = sizeof(GoodCheck<Base>(0)) == sizeof(Yes);
-//
-// Notice here that the variadic version of GoodCheck() returns Yes here
-// instead of No like the previous one. Also notice that we calculate |value|
-// by specializing GoodCheck() on Base instead of T.
-//
-// We've reversed the roles of the variadic, and Helper overloads.
-// GoodCheck(Helper<&C::TargetFunc>*), when C = Base, fails to be a valid
-// substitution if T::TargetFunc exists. Thus GoodCheck<Base>(0) will resolve
-// to the variadic version if T has TargetFunc.  If T::TargetFunc does not
-// exist, then &C::TargetFunc is not ambiguous, and the overload resolution
-// will prefer GoodCheck(Helper<&C::TargetFunc>*).
-//
-// This method of SFINAE will correctly probe for inherited names, but it cannot
-// typecheck those names.  It's still a good enough sanity check though.
-//
-// Works on gcc-4.2, gcc-4.4, and Visual Studio 2008.
-//
-
-template <typename T>
-class HasIsMethodTag {
-  using Yes = char[1];
-  using No = char[2];
-
-  template <typename U>
-  static Yes& Check(typename U::IsMethod*);
-
-  template <typename U>
-  static No& Check(...);
-
- public:
-  enum { value = sizeof(Check<T>(0)) == sizeof(Yes) };
-};
-
 template <typename T>
 class UnretainedWrapper {
  public:
@@ -287,6 +207,7 @@ class RetainedRefWrapper {
 template <typename T>
 struct IgnoreResultHelper {
   explicit IgnoreResultHelper(T functor) : functor_(std::move(functor)) {}
+  explicit operator bool() const { return !!functor_; }
 
   T functor_;
 };