Add SERP metrics prefs target (#34712)

The PR adds SERP metrics prefs target and includes
kDeprecatedSerpMetricsTimePeriodStorage preference to it.
This will allow us to use kDeprecatedSerpMetricsTimePeriodStorage
preference without duplication. The change is pre-requisite for
SERP metrics profile attribute migration.

browser/brave_profile_prefs.cc

@@ -6,7 +6,6 @@
 #include "brave/browser/brave_profile_prefs.h"
 
 #include <string>
-#include <string_view>
 
 #include "base/feature_list.h"
 #include "brave/browser/brave_browser_features.h"
@@ -53,6 +52,7 @@
 #include "brave/components/query_filter/pref_names.h"
 #include "brave/components/request_otr/common/buildflags/buildflags.h"
 #include "brave/components/search_engines/brave_prepopulated_engines.h"
+#include "brave/components/serp_metrics/pref_names.h"
 #include "brave/components/speedreader/common/buildflags/buildflags.h"
 #include "brave/components/tor/buildflags/buildflags.h"
 #include "brave/components/web_discovery/buildflags/buildflags.h"
@@ -176,10 +176,6 @@ namespace brave {
 
 namespace {
 
-// Deprecated 2026-03.
-constexpr std::string_view kDeprecatedSerpMetricsTimePeriodStorage =
-    "brave.stats.serp_metrics";
-
 void OverrideDefaultPrefValues(user_prefs::PrefRegistrySyncable* registry) {
 #if BUILDFLAG(IS_ANDROID)
   // Clear default popular sites
@@ -389,7 +385,8 @@ void RegisterProfilePrefsForMigration(
   registry->RegisterBooleanPref(prefs::kAddOpenSearchEngines, false);
 
   // Added 2026-03
-  registry->RegisterDictionaryPref(kDeprecatedSerpMetricsTimePeriodStorage);
+  registry->RegisterDictionaryPref(
+      serp_metrics::prefs::kDeprecatedSerpMetricsTimePeriodStorage);
 }
 
 void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry) {

browser/sources.gni

@@ -239,6 +239,7 @@ brave_chrome_browser_deps = [
   "//brave/components/request_otr/common/buildflags",
   "//brave/components/resources",
   "//brave/components/serp_metrics",
+  "//brave/components/serp_metrics:prefs",
   "//brave/components/skus/browser",
   "//brave/components/skus/common",
   "//brave/components/skus/common:mojom",

components/serp_metrics/BUILD.gn

@@ -39,6 +39,10 @@ static_library("features") {
   public_deps = [ "//base" ]
 }
 
+source_set("prefs") {
+  public = [ "pref_names.h" ]
+}
+
 source_set("unit_tests") {
   testonly = true
 

components/serp_metrics/pref_names.h

@@ -0,0 +1,19 @@
+/* Copyright (c) 2026 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#ifndef BRAVE_COMPONENTS_SERP_METRICS_PREF_NAMES_H_
+#define BRAVE_COMPONENTS_SERP_METRICS_PREF_NAMES_H_
+
+#include <string_view>
+
+namespace serp_metrics::prefs {
+
+// Deprecated 2026-03.
+inline constexpr std::string_view kDeprecatedSerpMetricsTimePeriodStorage =
+    "brave.stats.serp_metrics";
+
+}  // namespace serp_metrics::prefs
+
+#endif  // BRAVE_COMPONENTS_SERP_METRICS_PREF_NAMES_H_