Convert TrackedPreferenceValidationDelegate into a mojo interface.

components/user_prefs/tracked/BUILD.gn

Index: components/user_prefs/tracked/BUILD.gn
diff --git a/components/user_prefs/tracked/BUILD.gn b/components/user_prefs/tracked/BUILD.gn
index d2108c617622e7f47000a8dc8f99d02bf1827e3f..c1022463af7cc0fec653fcd2e0689d02eed7cd72 100644
--- a/components/user_prefs/tracked/BUILD.gn
+++ b/components/user_prefs/tracked/BUILD.gn
@@ -34,7 +34,6 @@ static_library("user_prefs_tracked") {
     "tracked_preference_helper.h",
     "tracked_preference_histogram_names.cc",
     "tracked_preference_histogram_names.h",
-    "tracked_preference_validation_delegate.h",
     "tracked_preferences_migration.cc",
     "tracked_preferences_migration.h",
     "tracked_split_preference.cc",
@@ -48,6 +47,10 @@ static_library("user_prefs_tracked") {
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
+  public_deps = [
+    "//services/preferences/public/interfaces",
+  ]
+
   deps = [
     "//base:base",
     "//components/pref_registry",