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/+/1862451Reviewed-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}

base/bind.h

@@ -191,14 +191,12 @@ template <bool is_once, typename Invoker>
 struct InvokeFuncImpl;
 
 template <typename Invoker>
-struct InvokeFuncImpl<true, Invoker> {
-  static constexpr auto Value = &Invoker::RunOnce;
-};
+struct InvokeFuncImpl<true, Invoker>
+    : std::integral_constant<decltype(&Invoker::RunOnce), &Invoker::RunOnce> {};
 
 template <typename Invoker>
-struct InvokeFuncImpl<false, Invoker> {
-  static constexpr auto Value = &Invoker::Run;
-};
+struct InvokeFuncImpl<false, Invoker>
+    : std::integral_constant<decltype(&Invoker::Run), &Invoker::Run> {};
 
 template <template <typename> class CallbackT,
           typename Functor,
@@ -229,7 +227,7 @@ 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 = InvokeFuncImpl<kIsOnce, Invoker>::value;
 
   using InvokeFuncStorage = internal::BindStateBase::InvokeFuncStorage;
   return CallbackType(BindState::Create(