Most of the settings for creating URLRequestContext are same and
we use same code in different functions. To reduce the redundant
code, we add a helper function called 'ConfigURLRequestContext'
to setup all configs of URLRequestContext object.

[1]: https://cs.chromium.org/chromium/src/chromecast/browser/url_request_context_factory.cc?l=396&rcl=9278acaca70fd85c7000969f5ecfec33a4badc17
[2]: https://cs.chromium.org/chromium/src/chromecast/browser/url_request_context_factory.cc?l=335&rcl=9278acaca70fd85c7000969f5ecfec33a4badc17