GCC build fix: base::Optional<T> requires the full declaration of T

In GCC 7.2/7.3, base::Optional<T> fails to compile, if T has still
only been forward declared, as it cannot properly resolve the
is_trivially_* declarations. In this case it is needed to include the
full declaration of the type, and not only the forward declaration.

Change-Id: I63e5c6307394c6c2eda6af108c80395152bfc04f
Reviewed-on: https://chromium-review.googlesource.com/944401
Commit-Queue: José Dapena Paz <jose.dapena@lge.com>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540815}

services/preferences/tracked/pref_hash_filter.h

@@ -21,9 +21,9 @@
 #include "services/preferences/public/mojom/preferences.mojom.h"
 #include "services/preferences/tracked/hash_store_contents.h"
 #include "services/preferences/tracked/interceptable_pref_filter.h"
+#include "services/preferences/tracked/pref_hash_store.h"
 #include "services/preferences/tracked/tracked_preference.h"
 
-class PrefHashStore;
 class PrefService;
 
 namespace base {