Reland [base] Require task posters to specify an explicit destination

This patch makes it a requirement for all task traits to specify their
thread affinity. Previously task traits that did not name a specific
thread would implicitly run on the thread pool, which could be
surprising. To make this more obvious, thread pool tasks must now use
the base::ThreadPool() trait.

This is the final reland of
https://chromium-review.googlesource.com/c/chromium/src/+/1635827
(will attempt to fix errors that caused original revert before
 relanding).

Bug: 968047
Change-Id: Ice339db270a27815b2be68200930e5bc105e8a96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1759056
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688782}

base/task/task_traits.h

@@ -255,7 +255,16 @@ class BASE_EXPORT TaskTraits {
         may_block_(trait_helpers::HasTrait<MayBlock, ArgTypes...>()),
         with_base_sync_primitives_(
             trait_helpers::HasTrait<WithBaseSyncPrimitives, ArgTypes...>()),
-        use_thread_pool_(trait_helpers::HasTrait<ThreadPool, ArgTypes...>()) {}
+        use_thread_pool_(trait_helpers::HasTrait<ThreadPool, ArgTypes...>()) {
+    constexpr bool has_thread_pool =
+        trait_helpers::HasTrait<ThreadPool, ArgTypes...>();
+    constexpr bool has_extension =
+        !trait_helpers::AreValidTraits<ValidTrait, ArgTypes...>::value;
+    static_assert(
+        has_thread_pool ^ has_extension,
+        "Traits must explicitly specify a destination (e.g. ThreadPool or a "
+        "named thread like BrowserThread)");
+  }
 
   constexpr TaskTraits(const TaskTraits& other) = default;
   TaskTraits& operator=(const TaskTraits& other) = default;