[FIX/IMP] export browser object, prevent crash in private mode

This commit performs two tasks:

1. it exports the internal browser object (in owl.browser), for ease of
testing
2. it makes sure that Owl does not crash inside iframes in incognito
mode, because window.localStorage raises an exception in that case

closes #791

doc/reference/browser.md

@@ -0,0 +1,33 @@
+# 🦉 Browser 🦉
+
+## Content
+
+- [Overview](#overview)
+- [Browser Content](#browser-content)
+
+## Overview
+
+The browser object contains some browser native APIs, such as `setTimeout`, that
+are used by Owl and its utility functions. They are exposed with the intent of
+making them mockable if necessary.
+
+```js
+owl.browser.setTimeout === window.setTimeout; // return true
+```
+
+For now, this object contains some functions that are not used by Owl. They
+will eventually be removed in Owl 2.0.
+
+## Browser Content
+
+More specifically, the `browser` object contains the following methods and objects:
+
+- `setTimeout`
+- `clearTimeout`
+- `setInterval`
+- `clearInterval`
+- `requestAnimationFrame`
+- `random`
+- `Date`
+- `fetch`
+- `localStorage`