move backend specific code out of AbstractTab into respective backend

classes.

qutebrowser/browser/browsertab.py

@@ -724,12 +724,6 @@ class AbstractTab(QWidget):
         self._progress = 0
         self._has_ssl_errors = False
         self.data.viewing_source = False
-        if self.backend == usertypes.Backend.QtWebKit:
-            obj = self.networkaccessmanager()
-        else:
-            obj = self
-        if hasattr(obj, 'netrc_used'):
-            delattr(obj, 'netrc_used')
         self._set_load_status(usertypes.LoadStatus.loading)
         self.load_started.emit()
 

qutebrowser/browser/webengine/webenginetab.py

@@ -783,6 +783,8 @@ class WebEngineTab(browsertab.AbstractTab):
             # https://bugreports.qt.io/browse/QTBUG-61506
             self.search.clear()
         super()._on_load_started()
+        if hasattr(self, 'netrc_used'):
+            delattr(self, 'netrc_used')
 
     @pyqtSlot(QWebEnginePage.RenderProcessTerminationStatus, int)
     def _on_render_process_terminated(self, status, exitcode):

qutebrowser/browser/webkit/webkittab.py

@@ -704,6 +704,13 @@ class WebKitTab(browsertab.AbstractTab):
         page = self._widget.page()
         return page.userAgentForUrl(self.url())
 
+    @pyqtSlot()
+    def _on_load_started(self):
+        super()._on_load_started()
+        obj = self.networkaccessmanager()
+        if hasattr(obj, 'netrc_used'):
+            delattr(obj, 'netrc_used')
+
     @pyqtSlot()
     def _on_frame_load_finished(self):
         """Make sure we emit an appropriate status when loading finished.