Add missing #include around sequenced_worker_pool.h in //chrome

This is a //chrome part of https://codereview.chromium.org/2690533002/ PS6.

These files depends on indirectly-included sequenced_worker_pool.h, which is
being removed.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win10_chromium_x64_rel_ng

Review-Url: https://codereview.chromium.org/2695823008
Cr-Commit-Position: refs/heads/master@{#450892}
diff --git a/chrome/browser/net/file_downloader.cc b/chrome/browser/net/file_downloader.cc
index aa57047..87ade7a9 100644
--- a/chrome/browser/net/file_downloader.cc
+++ b/chrome/browser/net/file_downloader.cc
@@ -8,6 +8,7 @@
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/base/load_flags.h"
 #include "net/http/http_status_code.h"