Fixes style in WebRequestContext

Fixes style in WebRequestContext

Previously this file used Chromium style, as it extends a Chromium
class. However, I think it's best to switch to WebKit style to prevent
Chromium style from leaking into the rest of WebKit.

Also switches static member methods to file-scope static functions
where possible.

Finally, return WebRequestContext* rather than URLRequestContext* to
allow us to call methods in the derived class. this will be required
to hook up CookieManager.

Change-Id: Ifdca62230a3728e338904ef1b9b392640af06a92