Bug 807998 - Turn on memory.free_dirty_pages in Fennec. r=jlebar

mobile/android/app/mobile.js

@@ -667,3 +667,7 @@ pref("dom.event.touch.coalescing.enabled", false);
 // default orientation for the app, default to undefined
 // the java GeckoScreenOrientationListener needs this to be defined
 pref("app.orientation.default", "");
+
+// On memory pressure, release dirty but unused pages held by jemalloc
+// back to the system.
+pref("memory.free_dirty_pages", true);