lacros: Fix DCHECK failure.

In base::Process::WaitForExitWithTimeout, it is checked whether
base sync primitives is allowed. Send to the thread with the trait.

Bug: 1103798
Test: Ran locally, and no DCHECK error on lacros-chrome termination.
Change-Id: Ie1d15d1ee949a6f7c06726ccb14c90b38fea96ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299947Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788888}

chrome/browser/chromeos/crosapi/lacros_manager.cc

@@ -241,7 +241,7 @@ void LacrosManager::OnMojoDisconnected() {
   lacros_chrome_service_.reset();
   ash_chrome_service_ = nullptr;
   base::ThreadPool::PostTaskAndReply(
-      FROM_HERE, {base::MayBlock()},
+      FROM_HERE, {base::WithBaseSyncPrimitives()},
       base::BindOnce(&TerminateLacrosChrome, std::move(lacros_process_)),
       base::BindOnce(&LacrosManager::OnLacrosChromeTerminated,
                      weak_factory_.GetWeakPtr()));