obs-browser: flush panel cookie store on exit

If cookie store is not flushed, state might become inconsistent:
reading from the cookies file will become impossible.
No errors will be returned, yet no information will be available,
and no information will be stored: the cookie manager will appear to be
empty at all times and the `Cookies` file modified date will not update
moving forward.

This will result, for example, in Twitch chat window re-opening for
users which connected their Twitch account, but not having the
`auth-token` cookie available: once the user will try to send a
message to the chat, they will be redirected to Twitch login screen.

obs-browser-plugin.cpp

@@ -60,6 +60,33 @@ bool hwaccel = false;
 
 /* ========================================================================= */
 
+static std::mutex cookie_managers_mutex;
+static std::vector<CefRefPtr<CefCookieManager>> cookie_managers;
+
+void register_cookie_manager(CefRefPtr<CefCookieManager> cm)
+{
+	std::lock_guard<std::mutex> guard(cookie_managers_mutex);
+
+	cookie_managers.emplace_back(cm);
+}
+
+static void flush_cookie_managers()
+{
+	std::lock_guard<std::mutex> guard(cookie_managers_mutex);
+
+	for (auto cm : cookie_managers) {
+		if (cm->FlushStore(nullptr)) {
+			blog(LOG_INFO, "Flushed cookie store");
+		}
+		else {
+			blog(LOG_WARNING,
+				"Failed flushing cookie store");
+		}
+	}
+}
+
+/* ========================================================================= */
+
 class BrowserTask : public CefTask {
 public:
 	std::function<void()> task;
@@ -500,6 +527,8 @@ bool obs_module_load(void)
 
 void obs_module_unload(void)
 {
+	flush_cookie_managers();
+
 	if (manager_thread.joinable()) {
 		while (!QueueCEFTask([] () {CefQuitMessageLoop();}))
 			os_sleep_ms(5);

panel/browser-panel.cpp

@@ -13,6 +13,8 @@ extern bool QueueCEFTask(std::function<void()> task);
 extern "C" void obs_browser_initialize(void);
 extern os_event_t *cef_started_event;
 
+void register_cookie_manager(CefRefPtr<CefCookieManager> cm);
+
 std::mutex                      popup_whitelist_mutex;
 std::vector<PopupWhitelistInfo> popup_whitelist;
 std::vector<PopupWhitelistInfo> forced_popups;
@@ -83,6 +85,8 @@ struct QCefCookieManagerInternal : QCefCookieManager {
 		if (!cm)
 			throw "Failed to create cookie manager";
 
+		register_cookie_manager(cm);
+
 		rch = new QCefRequestContextHandler(cm);
 
 		rc = CefRequestContext::CreateContext(