fix: misc-use-internal-linkage warnings (#44872)

* refactor: fix misc-use-internal-linkage warnings:

move impl functions into anonymous namespace so that they're not visible
to other compilation units

shell/common/platform_util.cc

@@ -13,6 +13,8 @@
 
 namespace platform_util {
 
+namespace {
+
 struct TrashItemResult {
   bool success;
   std::string error;
@@ -24,6 +26,8 @@ TrashItemResult TrashItemOnBlockingThread(const base::FilePath& full_path) {
   return {success, error};
 }
 
+}  // namespace
+
 void TrashItem(const base::FilePath& full_path,
                base::OnceCallback<void(bool, const std::string&)> callback) {
   // XXX: is continue_on_shutdown right?