Reland "GCC: Fix base::internal::InvokeFuncImpl"

This is a reland of 9293d5c8

Original change's description:
> GCC: Fix base::internal::InvokeFuncImpl
> 
> GCC doesn't like that the Value data member has no out-of-line
> definition. The problem is triggered specifically only when compiling
> 
>   components/services/leveldb/leveldb_database_impl.cc
> 
> which has lambda functions returning locally-defined classes.
> 
> The current code works as-is in C++17 mode which introduces the concept
> of inline variables, but in C++14 we need either an explicit out-of-line
> definition or a function member instead of a data member.
> 
> Use std::integral_constant for defining the value.
> 
> Bug: 819294
> Change-Id: I5c68e14ce3fa9d8b4d8a2cb42d7f9b53938aabf3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862451
> Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Jüri Valdmann <juri.valdmann@qt.io>
> Cr-Commit-Position: refs/heads/master@{#706384}

Bug: 819294
Change-Id: I3d5a52ddc6815516e2239f9347c60de06bf765a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865212Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707329}

base/bind.h

@@ -187,18 +187,15 @@ using MakeUnwrappedTypeList =
 // well-formed. Using `Invoker::Run` with a OnceCallback triggers a
 // static_assert, which is why the ternary expression does not compile.
 // TODO(crbug.com/752720): Remove this indirection once we have `if constexpr`.
-template <bool is_once, typename Invoker>
-struct InvokeFuncImpl;
-
 template <typename Invoker>
-struct InvokeFuncImpl<true, Invoker> {
-  static constexpr auto Value = &Invoker::RunOnce;
-};
+constexpr auto GetInvokeFunc(std::true_type) {
+  return Invoker::RunOnce;
+}
 
 template <typename Invoker>
-struct InvokeFuncImpl<false, Invoker> {
-  static constexpr auto Value = &Invoker::Run;
-};
+constexpr auto GetInvokeFunc(std::false_type) {
+  return Invoker::Run;
+}
 
 template <template <typename> class CallbackT,
           typename Functor,
@@ -229,7 +226,8 @@ decltype(auto) BindImpl(Functor&& functor, Args&&... args) {
   // InvokeFuncStorage, so that we can ensure its type matches to
   // PolymorphicInvoke, to which CallbackType will cast back.
   using PolymorphicInvoke = typename CallbackType::PolymorphicInvoke;
-  PolymorphicInvoke invoke_func = InvokeFuncImpl<kIsOnce, Invoker>::Value;
+  PolymorphicInvoke invoke_func =
+      GetInvokeFunc<Invoker>(std::integral_constant<bool, kIsOnce>());
 
   using InvokeFuncStorage = internal::BindStateBase::InvokeFuncStorage;
   return CallbackType(BindState::Create(