update_engine: Update libchrome APIs to r369476

The new libchrome has been ported from Chromium and some APIs have
changed. Make necessary changes at call sites.

Change-Id: I42e65bda7f1dbdf6f6e0ebf356d2cfea6b729193
diff --git a/omaha_request_action_unittest.cc b/omaha_request_action_unittest.cc
index 69585c8..e5cc7e2 100644
--- a/omaha_request_action_unittest.cc
+++ b/omaha_request_action_unittest.cc
@@ -1073,8 +1073,8 @@
   fake_prefs_.SetString(kPrefsOmahaCohort, "evil\nstring");
   fake_prefs_.SetString(kPrefsOmahaCohortHint, "evil&string\\");
   fake_prefs_.SetString(kPrefsOmahaCohortName,
-                        JoinString(vector<string>(100, "My spoon is too big."),
-                                   ' '));
+                        base::JoinString(
+                            vector<string>(100, "My spoon is too big."), " "));
   OmahaResponse response;
   ASSERT_FALSE(
       TestUpdateCheck(&params,