Include <functional> in headers that reference struct hash.

IWYU. This is more of https://crrev.com/c/1536740

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Change-Id: I113c9c35ebab9b4d9d6b461d7fb0880a8e53d1e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536763
Reviewed-by: Lei Zhang <[email protected]>
Reviewed-by: Bill Budge <[email protected]>
Reviewed-by: Mounir Lamouri <[email protected]>
Commit-Queue: Lei Zhang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#645623}
diff --git a/components/prefs/pref_value_store.h b/components/prefs/pref_value_store.h
index 3d5676eb..70a92bc 100644
--- a/components/prefs/pref_value_store.h
+++ b/components/prefs/pref_value_store.h
@@ -5,6 +5,7 @@
 #ifndef COMPONENTS_PREFS_PREF_VALUE_STORE_H_
 #define COMPONENTS_PREFS_PREF_VALUE_STORE_H_
 
+#include <functional>
 #include <map>
 #include <memory>
 #include <string>