Need a system URLRequestContextGetter

[Olivier Tilloy]

While working on bug #1267543 to implement an AccessTokenStore for use in geolocation services, I was confronted with the need to pass a URLRequestContextGetter* to the callback in AccessTokenStore::LoadAccessTokens(…).

The way it’s implemented in Chrome is by exposing a system request context (see https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/geolocation/chrome_access_token_store.cc&l=87 and https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/browser_process.h&l=105).

We are going to need a similar mechanism in oxide.