[embedlite] Enable meta viewport support.

The pref was added somewhere between v31 and v38. It's enabled by
default on b2g and fennect. The code that the feature controls was
enabled by default in enbedlite_31. Without meta viewport support most
mobile pages are broken and multitouch gestures don't work.

embedding/embedlite/embedding.js

@@ -1,4 +1,5 @@
 pref("dom.w3c_touch_events.enabled", 1);
+pref("dom.meta-viewport.enabled", true);
 pref("plugins.force.wmode", "opaque");
 pref("browser.xul.error_pages.enabled", true);
 pref("nglayout.debug.paint_flashing", false);