Add a workaround to save cookies on exit

Fixes #2263

CHANGELOG.asciidoc

@@ -57,6 +57,8 @@ Fixed
 - Crash reports are now re-enabled when using QtWebEngine
 - Fixed crashes when closing tabs while hinting
 - Fixed starting on newer PyQt/sip versions with LibreSSL
+- Worked around a Qt bug which caused cookies newer than 30s not to be saved on
+  exit with QtWebEngine.
 
 v0.9.1
 ------

qutebrowser/browser/webengine/webenginesettings.py

@@ -26,6 +26,7 @@ Module attributes:
 
 import os
 
+from PyQt5.QtNetwork import QNetworkCookie
 # pylint: disable=no-name-in-module,import-error,useless-suppression
 from PyQt5.QtWebEngineWidgets import (QWebEngineSettings, QWebEngineProfile,
                                       QWebEngineScript)
@@ -171,8 +172,11 @@ def init(args):
 
 
 def shutdown():
-    # FIXME:qtwebengine do we need to do something for a clean shutdown here?
-    pass
+    # WORKAROUND for https://bugreports.qt.io/browse/QTBUG-58675
+    # This forces Chromium to flush its cookie store but doesn't actually
+    # delete anything.
+    cookie = QNetworkCookie()
+    QWebEngineProfile.defaultProfile().cookieStore().deleteCookie(cookie)
 
 
 # Missing QtWebEngine attributes: