[remoting host][remote open url] Fix bug when XFCE fallback browser i…

…s not found

Currently if the configure-url-forwarder script finds that the default
browser of Cinnamon is the XFCE browser entry, then it will look for
the XFCE default browser and make it the default browser for Cinnamon.
This generally works fine, but it may raise a key-not-found exception in
rare cases like the user has set up the URL forwarder then they somehow
lose the host settings file.

This CL simply makes the script skip overriding the default browser if
the XFCE browser entry is not found. This would keep the fallback
browser "xfce4-web-browser.desktop" for Cinnamon, but after
crrev.com/c/2977713, the URL forwarder will detect the problem and ask
the user to choose the fallback browser.

Bug: b:183135000
Change-Id: Iee98e4af9ab4e8a54f0d3d15b2653424cf7f30ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2981073
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#895342}

remoting/host/linux/configure_url_forwarder.py

@@ -163,7 +163,11 @@ def setup_url_forwarder() -> None:
               XFCE4_WEB_BROWSER_DESKTOP_ENTRY, 'but XFCE is not found',
               file=sys.stderr)
         break
-      settings[setting_key] = settings[desktop_envs_and_setting_keys['XFCE']]
+      xfce_setting_key = desktop_envs_and_setting_keys['XFCE']
+      if xfce_setting_key not in settings:
+        print('Cannot find', xfce_setting_key, 'in host settings.')
+        break
+      settings[setting_key] = settings[xfce_setting_key]
 
   save_host_settings_file(settings)