Bug 507694: Change JS GC frequency and high-watermark preferences for mobile, r=stuart

mobile/app/mobile.js

@@ -319,6 +319,7 @@ pref("content.max.tokenizing.time", 50000);
 
 pref("javascript.options.jit.content", true);
 pref("javascript.options.jit.chrome", true);
+pref("javascript.options.mem.gc_frequency", 300);
 
 pref("dom.max_chrome_script_run_time", 30);
 pref("dom.max_script_run_time", 20);