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.
Sorry, I'm a little confused. Do you mean I just put this tag, <span data-l10n-id="dumps-layerscope"></span>, and don't need to write down "Dump layerscope" in the html file anymore, or have to remove this tag? Thanks.