config: Whitelists the desktopSharingFrameRate config key.

react/features/base/config/functions.js

     'desktopSharingChromeExtId',
     'desktopSharingChromeMinExtVersion',
     'desktopSharingChromeSources',
+    'desktopSharingFrameRate',
     'desktopSharingFirefoxDisabled',
     'desktopSharingSources',
     'disable1On1Mode',