[Cleanup] Add a missing #include.

R=asvitkine@chromium.org

Bug: none
Change-Id: I4e256bef6d7cc6f3af24a5b1241d2279eaf82cad
Reviewed-on: https://chromium-review.googlesource.com/813499
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522418}

components/variations/client_filterable_state.h

@@ -5,6 +5,8 @@
 #ifndef COMPONENTS_VARIATIONS_CLIENT_FILTERABLE_STATE_H_
 #define COMPONENTS_VARIATIONS_CLIENT_FILTERABLE_STATE_H_
 
+#include <string>
+
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "base/version.h"