Stop relying on linking details of std’s default allocator

We’ve been bitten before by symbol names changing:
https://github.com/servo/heapsize/pull/46
and upstream is planning to stop using jemalloc by default:
https://github.com/rust-lang/rust/issues/33082#issuecomment-309781465

So use the (relatively) new `#[global_allocator]` attribute
to explicitly select the system allocator on Windows
and jemalloc (now in an external crate) on other platforms.
This choice matches current defaults.

components/script/Cargo.toml

@@ -13,7 +13,7 @@ path = "lib.rs"
 
 [features]
 debugmozjs = ['js/debugmozjs']
-unstable = []
+unstable = ["servo_allocator/unstable"]
 
 [build-dependencies]
 cmake = "0.1"
@@ -77,6 +77,7 @@ script_plugins = {path = "../script_plugins"}
 script_traits = {path = "../script_traits"}
 selectors = { path = "../selectors" }
 serde = "1.0"
+servo_allocator = {path = "../allocator"}
 servo_arc = {path = "../servo_arc"}
 servo_atoms = {path = "../atoms"}
 servo_config = {path = "../config"}

components/script/lib.rs

@@ -78,6 +78,7 @@ extern crate script_layout_interface;
 extern crate script_traits;
 extern crate selectors;
 extern crate serde;
+extern crate servo_allocator;
 extern crate servo_arc;
 #[macro_use] extern crate servo_atoms;
 extern crate servo_config;

components/script/script_thread.rs

@@ -73,7 +73,7 @@ use js::jsapi::{JSAutoCompartment, JSContext, JS_SetWrapObjectCallbacks};
 use js::jsapi::{JSTracer, SetWindowProxyClass};
 use js::jsval::UndefinedValue;
 use js::rust::Runtime;
-use malloc_size_of::{malloc_size_of, MallocSizeOfOps};
+use malloc_size_of::MallocSizeOfOps;
 use mem::malloc_size_of_including_self;
 use metrics::PaintTimeMetrics;
 use microtask::{MicrotaskQueue, Microtask};
@@ -1506,7 +1506,7 @@ impl ScriptThread {
         let mut reports = vec![];
         // Servo uses vanilla jemalloc, which doesn't have a
         // malloc_enclosing_size_of function.
-        let mut ops = MallocSizeOfOps::new(malloc_size_of, None, None);
+        let mut ops = MallocSizeOfOps::new(::servo_allocator::usable_size, None, None);
 
         for (_, document) in self.documents.borrow().iter() {
             let current_url = document.url();