This patch adds a basic source map support to test runner.
SourceMap support is powered by Chromium DevTools source map
implementation (thus copyright). Unlike popular `source-map` npm
module, it's sync and pretty straight-forward.
The `SourceMap.js` file has a few modifications wrt upstream
Chromium version:
- reverse mappings API is removed. There's no need to ever compute them
- the `upperBoundary` function from DevTools' platform is inlined
