fixes #70 -- move d8 specific code into d8_benchmark.js

Also fixes the benchmark to work again on d8 ("navigator" was undefined). I ran it with:

```sh
d8 --allow-natives-syntax ../src/observe.js benchmark.js observation_benchmark.js d8_benchmarks.js
```

The main reason for this change is to avoid an exception on startup in Chrome (eval %RunMicrotasks will fail).