[base] Require task posters to specify an explicit destination on ChromeOs

This patch makes it a requirement for all task traits to specify their
thread affinity on ChromeOs. 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.

Previous patch applied the checks on all platforms except ChromeOs which was failing:
https://chromium-review.googlesource.com/c/chromium/src/+/1769080
This CL enables the check on ChromeOs:
joelhockey@ fixed the chromeos build in
https://chromium-review.googlesource.com/c/chromium/src/+/1767447

Bug: 968047
Change-Id: I18efe212b94fe35f55e446c76ce2c3f20cc259f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771976Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690825}

base/task/task_traits.h

@@ -256,10 +256,6 @@ class BASE_EXPORT TaskTraits {
         with_base_sync_primitives_(
             trait_helpers::HasTrait<WithBaseSyncPrimitives, ArgTypes...>()),
         use_thread_pool_(trait_helpers::HasTrait<ThreadPool, ArgTypes...>()) {
-#if !defined(OS_CHROMEOS)
-    // TODO(https://crbug.com/968047): Enable on Chrome OS in a separate CL.
-    // Initially disabled to keep the rule enforced on other platforms if the
-    // CL that enables it on Chrome OS is reverted.
     constexpr bool has_thread_pool =
         trait_helpers::HasTrait<ThreadPool, ArgTypes...>();
     constexpr bool has_extension =
@@ -268,7 +264,6 @@ class BASE_EXPORT TaskTraits {
         has_thread_pool ^ has_extension,
         "Traits must explicitly specify a destination (e.g. ThreadPool or a "
         "named thread like BrowserThread)");
-#endif
   }
 
   constexpr TaskTraits(const TaskTraits& other) = default;