feat(ScreenSharing): use sane defaults

Both Chrome and Firefox can work without extensions now, so it no longer makes
sense to default to disabling it in Chrome. Moreover, rely on the fact that
undefined is falsey so no actual config needs to be provided.

config.js

@@ -146,7 +146,7 @@ var config = {
     desktopSharingChromeExtId: null,
 
     // Whether desktop sharing should be disabled on Chrome.
-    desktopSharingChromeDisabled: true,
+    // desktopSharingChromeDisabled: false,
 
     // The media sources to use when using screen sharing with the Chrome
     // extension.
@@ -156,7 +156,7 @@ var config = {
     desktopSharingChromeMinExtVersion: '0.1',
 
     // Whether desktop sharing should be disabled on Firefox.
-    desktopSharingFirefoxDisabled: false,
+    // desktopSharingFirefoxDisabled: false,
 
     // Optional desktop sharing frame rate options. Default value: min:5, max:5.
     // desktopSharingFrameRate: {