Bug 1040651 - Map a preference to a Gaia setting to enable Layerscope. r=fabrice

Add a listener at Gecko side. We will also add a "LayerScope" option
in "Settings -> Developer", so we can enable Layerscope dynamically.

b2g/chrome/content/settings.js

@@ -533,6 +533,7 @@ let settingsToObserve = {
   },
   'dom.mozApps.use_reviewer_certs': false,
   'dom.mozApps.signed_apps_installable_from': 'https://marketplace.firefox.com',
+  'gfx.layerscope.enabled': false,
   'layers.draw-borders': false,
   'layers.draw-tile-borders': false,
   'layers.dump': false,