Fix IWYU issue from https://chromium-review.googlesource.com/c/578572/ .

Bug: None
Change-Id: I8b2e886fcb7bc2d47f149f3628fa52b92f74f0ee
TBR: gab@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/580644Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488588}

components/webdata/common/web_data_request_manager.h

@@ -15,6 +15,7 @@
 #include "base/atomicops.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/sequenced_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "components/webdata/common/web_data_results.h"
 #include "components/webdata/common/web_data_service_base.h"
@@ -24,10 +25,6 @@
 class WebDataServiceConsumer;
 class WebDataRequestManager;
 
-namespace base {
-class SequencedTaskRunner;
-}
-
 //////////////////////////////////////////////////////////////////////////////
 //
 // WebData requests