Revert of Move JoinString to the base namespace

components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc

Index: components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc
diff --git a/components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc b/components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc
index 3544f4db5d9e40b49e51a9d613a2cfdc35f8676b..d6b95e995dbf688cb22182596d8a25fb632408f7 100644
--- a/components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc
+++ b/components/autofill/content/renderer/password_form_conversion_utils_browsertest.cc
@@ -269,8 +269,7 @@
         EXPECT_EQ(base::UTF8ToUTF16(cases[i].expected_username_value),
                   password_form->username_value);
         EXPECT_EQ(base::UTF8ToUTF16(cases[i].expected_other_possible_usernames),
-                  base::JoinString(password_form->other_possible_usernames,
-                                   base::ASCIIToUTF16("+")));
+                  JoinString(password_form->other_possible_usernames, '+'));
       } else {
         EXPECT_TRUE(password_form->username_value.empty());
         EXPECT_TRUE(password_form->other_possible_usernames.empty());