Convert usage of banned std::to_string() to base::IntToString().

R=oshima@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/795423002

Cr-Commit-Position: refs/heads/master@{#308049}

athena/wm/window_list_provider_impl_unittest.cc

@@ -11,6 +11,7 @@
 #include "athena/test/base/test_windows.h"
 #include "athena/wm/public/window_list_provider_observer.h"
 #include "athena/wm/public/window_manager.h"
+#include "base/strings/string_number_conversions.h"
 #include "ui/aura/client/window_tree_client.h"
 #include "ui/aura/test/test_window_delegate.h"
 #include "ui/aura/window.h"
@@ -50,7 +51,7 @@ std::string GetWindowOrder(const aura::Window::Windows& original,
     for (size_t i = 0; i < original.size(); i++) {
       if ((*it) == original[i]) {
         output += (output.size() ? " " : std::string()) +
-                  std::to_string(i + 1);
+                  base::IntToString(i + 1);
         break;
       }
     }