Deprecate 'singleton' option in chrome.apps.window.create().

chrome/browser/extensions/api/app_window/app_window_api.cc

Index: chrome/browser/extensions/api/app_window/app_window_api.cc
diff --git a/chrome/browser/extensions/api/app_window/app_window_api.cc b/chrome/browser/extensions/api/app_window/app_window_api.cc
index 0eaaaf58a1f426ad1c2e0985aca4e2eba5a4a8d6..671bc7145035c77e3b0c2f4fb4e88bdc469921bc 100644
--- a/chrome/browser/extensions/api/app_window/app_window_api.cc
+++ b/chrome/browser/extensions/api/app_window/app_window_api.cc
@@ -159,6 +159,13 @@ bool AppWindowCreateFunction::RunImpl() {
 
       create_params.window_key = *options->id;
 
+      if (options->singleton && *options->singleton == false) {
+        WriteToConsole(
+          content::CONSOLE_MESSAGE_LEVEL_WARNING,
+          "The 'singleton' option in chrome.apps.window.create() is deprecated!"
+          " Change your code to no longer rely on this.");
+      }
+
       if (!options->singleton || *options->singleton) {
         ShellWindow* window = apps::ShellWindowRegistry::Get(
             GetProfile())->GetShellWindowForAppAndKey(extension_id(),