chrome: Add callback for already running app relaunch (fixes #3609)

Adds a new CefBrowserProcessHandler::OnAlreadyRunningAppRelaunch
callback for when an already running app is relaunched with the
same CefSettings.root_cache_path.

Client apps should check the CefInitialize() return value for early
exit of the relaunch source process.

patch/patch.cfg

@@ -279,6 +279,11 @@ patches = [
     # https://github.com/chromiumembedded/cef/issues/3581
     'name': 'chrome_browser_policy',
   },
+  {
+    # Support override of Chrome process singleton behavior.
+    # https://github.com/chromiumembedded/cef/issues/3609
+    'name': 'chrome_browser_startup',
+  },
   {
     # alloy: Don't initialize ExtensionSystemFactory when extensions are
     # disabled.