Remove MessageLoop references in /components/offline_pages

MessageLoop is going away soon.

This CL was uploaded by git cl split.

R=dewittj@chromium.org

Bug: 891670
Change-Id: I23c7baa0f9429ba704a6ebfb356542eeb0cc8ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943185
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720550}

components/offline_pages/core/prefetch/prefetch_request_test_base.h

@@ -6,7 +6,6 @@
 #define COMPONENTS_OFFLINE_PAGES_CORE_PREFETCH_PREFETCH_REQUEST_FETCHER_TEST_BASE_H_
 
 #include "base/memory/ref_counted.h"
-#include "base/message_loop/message_loop.h"
 #include "base/metrics/field_trial.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/test/task_environment.h"