servo: Merge #10445 - Use better JS engine defaults (from metajack:enable-asmjs); r=jdm

This adds in preferences for all the SM 39 available options (as
retrieved from Gecko), and uses the same defaults as Gecko. A few
properties are not supported yet, and incremental GC is still always
disabled regardless of the preference setting.

Source-Repo: https://github.com/servo/servo
Source-Revision: bc2237ea2ba3beac501ca4347b8118f3dccd9629

servo/components/script/script_thread.rs

@@ -52,7 +52,8 @@ use hyper::mime::{Mime, SubLevel, TopLevel};
 use ipc_channel::ipc::{self, IpcSender};
 use ipc_channel::router::ROUTER;
 use js::jsapi::{DOMProxyShadowsResult, HandleId, HandleObject, RootedValue};
-use js::jsapi::{JSAutoRequest, JSContext, JS_SetWrapObjectCallbacks, JSTracer};
+use js::jsapi::{JSAutoRequest, JS_SetWrapObjectCallbacks};
+use js::jsapi::{JSContext, JSTracer};
 use js::jsval::UndefinedValue;
 use js::rust::Runtime;
 use layout_interface::{ReflowQueryType};