Add sanitize script

[camillobruni]

This script will eventually replace the benchmark-specifics overrides we already employ for some workloads.

• clamp all setTimeout to 0ms delay to limit effects of CPU throttling
• Make requestIdleCallback and cancelIdleCallback noops
• Set a fixed userAgent string to avoid running potentially browser-specific code

The sanitize.js script is supposed to be included in all benchmarks.