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This is intended to make servicification a little simpler, since we won't
have to configure a secret global URLRequestContext used just to fetch PAC
scripts.

The system URLRequestContext will be updated in a similar manner in a
followup CL.

BUG=715697

Review-Url: https://codereview.chromium.org/2841163002
Cr-Commit-Position: refs/heads/master@{#469044}