Bug 1040651 - Map a preference to a Gaia setting to enable Layerscope

Add a new option, "LayerScope", in "Settings -> Developer", so we can
enable this perference dynamically, instead of enabling it manually by
edit-prefs.sh.
Merge pull request #25976 from BorisChiou/layerscope

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