Reduce overhead of JavaScript calls

In the web our layout state calculation is actually massively dominated
by the calls to `performance.now()`. However before we can even call
`.now()` on the `performance` object, we first need to even get that. So
you first have to get the `window` object and access its `performance`
field. So overall that's three calls into JavaScript. If we however
cache the `performance` object, we can reduce it to just a single call.
That seems to improve the layout state calculation performance by over
2x.

Optimally we would only take a single time stamp per layout state
calculation, as otherwise there might be slight disagreements between
the individual values shown in the layout state. That's however a much
bigger refactoring, so that's something we can look into in the future.

Author: CryZe

src/platform/wasm/web/time.rs

@@ -6,13 +6,16 @@ use web_sys::window;
 #[derive(Copy, Clone, PartialOrd, PartialEq, Ord, Eq, Debug)]
 pub struct Instant(OrderedFloat<f64>);
 
+thread_local! {
+    static PERFORMANCE: web_sys::Performance = window()
+        .expect("No window object available")
+        .performance()
+        .expect("Can't measure time without a performance object");
+}
+
 impl Instant {
     pub fn now() -> Self {
-        let seconds = window()
-            .and_then(|w| w.performance())
-            .expect("Can't measure time without a performance object")
-            .now()
-            / 1000.0;
+        let seconds = PERFORMANCE.with(|p| p.now()) / 1000.0;
         Instant(OrderedFloat(seconds))
     }
 }