Remove UserPolicyRequestContext.
The request context mirrors the contents of the Profile's request
context, but uses "Accept-Language: *", which is not actually needed.
It also has a TODO about using the SystemURLRequestContext's proxy
service. This wouldn't actually have worked, since it shares the
Profile's context's HttpNetworkSession, which is where most of the proxy
logic lives, and the HttpNetworkSession has its own pointer to the
ProxyService it's using.
The main motivation for removing this class is that the Profile's
URLRequestContextGetter has recently been modified to safely shut down
URLFetchers that were use it during Profile shutdown, so just using
the Profile's URLRequestContextGetter directly will result in safer
shutdown.
BUG=488627
Review URL: https://codereview.chromium.org/
1140263002
Cr-Commit-Position: refs/heads/master@{#331368}