Fix media_router compile error in Chrome OS branded builds

kMediaRouterEnableCloudServices was missing a namespace in one file.

Tbr: khorimoto@chromium.org
Bug: none
Change-Id: Ia2abe411046f991893c2e8349d0424d3d7754123
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2431778Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810734}

chrome/browser/extensions/api/settings_private/prefs_util.cc

@@ -717,7 +717,7 @@ const PrefsUtil::TypedPrefMap& PrefsUtil::GetAllowlistedKeys() {
       settings_api::PrefType::PREF_TYPE_DICTIONARY;
 
 #if BUILDFLAG(GOOGLE_CHROME_BRANDING)
-  (*s_allowlist)[prefs::kMediaRouterEnableCloudServices] =
+  (*s_allowlist)[media_router::prefs::kMediaRouterEnableCloudServices] =
       settings_api::PrefType::PREF_TYPE_BOOLEAN;
   (*s_allowlist)[::prefs::kUserFeedbackAllowed] =
       settings_api::PrefType::PREF_TYPE_BOOLEAN;